Fix warnings on release builds (not DEBUG_ENABLED)

Fixes the following Clang 5 warnings:
```
modules/bmp/image_loader_bmp.cpp:46:60: warning: comparison of unsigned expression < 0 is always false [-Wtautological-compare]
modules/bmp/image_loader_bmp.cpp:48:61: warning: comparison of unsigned expression < 0 is always false [-Wtautological-compare]
drivers/png/image_loader_png.cpp:231:20: warning: comparison of unsigned expression >= 0 is always true [-Wtautological-compare]
scene/gui/graph_edit.cpp:1045:8: warning: comparison of constant 0 with expression of type 'bool' is always false [-Wtautological-constant-out-of-range-compare]

core/class_db.cpp:812:13: warning: unused variable 'check' [-Wunused-variable]
core/io/file_access_pack.cpp:172:11: warning: unused variable 'ver_rev' [-Wunused-variable]
core/math/bsp_tree.cpp:195:13: warning: unused variable 'plane' [-Wunused-variable]
core/math/bsp_tree.cpp:168:6: warning: unused variable 'plane_count' [-Wunused-variable]
modules/gdscript/gdscript_function.cpp:685:10: warning: unused variable 'ok' [-Wunused-variable]
modules/gdscript/gdscript_function.cpp:706:10: warning: unused variable 'ok' [-Wunused-variable]
modules/gdscript/gdscript_function.cpp:755:19: warning: unused variable 'var_type' [-Wunused-variable]
modules/gdscript/gdscript_function.cpp:1306:12: warning: unused variable 'err' [-Wunused-variable]

modules/gdscript/gdscript_function.cpp:158:15: warning: unused function '_get_var_type' [-Wunused-function]
modules/gdscript/gdscript_parser.cpp:750:20: warning: unused variable 'lv' [-Wunused-variable]
modules/gdscript/gdscript_parser.cpp:59:15: warning: unused function '_find_function_name' [-Wunused-function]
scene/main/node.cpp:2489:13: warning: unused function '_Node_debug_sn' [-Wunused-function]
```

3 files changed, 17 insertions, 20 deletions

diff --git a/modules/bmp/image_loader_bmp.cpp b/modules/bmp/image_loader_bmp.cpp
index 919731b52b..063508a25f 100644
--- a/modules/bmp/image_loader_bmp.cpp
+++ b/modules/bmp/image_loader_bmp.cpp
@@ -42,12 +42,9 @@ Error ImageLoaderBMP::convert_to_image(Ref<Image> p_image,
 
 	if (err == OK) {
 		size_t index = 0;
-		size_t width =
-				static_cast<size_t>(p_header.bmp_info_header.bmp_width < 0 ? -p_header.bmp_info_header.bmp_width : p_header.bmp_info_header.bmp_width);
-		size_t height =
-				static_cast<size_t>(p_header.bmp_info_header.bmp_height < 0 ? -p_header.bmp_info_header.bmp_height : p_header.bmp_info_header.bmp_height);
-		size_t bits_per_pixel =
-				static_cast<size_t>(p_header.bmp_info_header.bmp_bit_count);
+		size_t width = (size_t)p_header.bmp_info_header.bmp_width;
+		size_t height = (size_t)p_header.bmp_info_header.bmp_height;
+		size_t bits_per_pixel = (size_t)p_header.bmp_info_header.bmp_bit_count;
 
 		if (p_header.bmp_info_header.bmp_compression != 0) {
 			err = FAILED;
diff --git a/modules/gdscript/gdscript_function.cpp b/modules/gdscript/gdscript_function.cpp
index b935861652..8088dcf17d 100644
--- a/modules/gdscript/gdscript_function.cpp
+++ b/modules/gdscript/gdscript_function.cpp
@@ -155,6 +155,7 @@ String GDScriptFunction::_get_call_error(const Variant::CallError &p_err, const
 	return err_text;
 }
 
+#ifdef DEBUG_ENABLED
 static String _get_var_type(const Variant *p_type) {
 
 	String basestr;
@@ -164,7 +165,6 @@ static String _get_var_type(const Variant *p_type) {
 		if (!bobj) {
 			basestr = "null instance";
 		} else {
-#ifdef DEBUG_ENABLED
 			if (ObjectDB::instance_validate(bobj)) {
 				if (bobj->get_script_instance())
 					basestr = bobj->get_class() + " (" + bobj->get_script_instance()->get_script()->get_path().get_file() + ")";
@@ -173,10 +173,6 @@ static String _get_var_type(const Variant *p_type) {
 			} else {
 				basestr = "previously freed instance";
 			}
-
-#else
-			basestr = "Object";
-#endif
 		}
 
 	} else {
@@ -185,6 +181,7 @@ static String _get_var_type(const Variant *p_type) {
 
 	return basestr;
 }
+#endif
 
 #if defined(__GNUC__)
 #define OPCODES_TABLE                         \
@@ -676,6 +673,7 @@ Variant GDScriptFunction::call(GDScriptInstance *p_instance, const Variant **p_a
 			OPCODE(OPCODE_SET_MEMBER) {
 
 				CHECK_SPACE(3);
+#ifdef DEBUG_ENABLED
 				int indexname = _code_ptr[ip + 1];
 				GD_ERR_BREAK(indexname < 0 || indexname >= _global_names_count);
 				const StringName *index = &_global_names_ptr[indexname];
@@ -683,7 +681,6 @@ Variant GDScriptFunction::call(GDScriptInstance *p_instance, const Variant **p_a
 
 				bool valid;
 				bool ok = ClassDB::set_property(p_instance->owner, *index, *src, &valid);
-#ifdef DEBUG_ENABLED
 				if (!ok) {
 					err_text = "Internal error setting property: " + String(*index);
 					OPCODE_BREAK;
@@ -699,13 +696,13 @@ Variant GDScriptFunction::call(GDScriptInstance *p_instance, const Variant **p_a
 			OPCODE(OPCODE_GET_MEMBER) {
 
 				CHECK_SPACE(3);
+#ifdef DEBUG_ENABLED
 				int indexname = _code_ptr[ip + 1];
 				GD_ERR_BREAK(indexname < 0 || indexname >= _global_names_count);
 				const StringName *index = &_global_names_ptr[indexname];
 				GET_VARIANT_PTR(dst, 2);
-				bool ok = ClassDB::get_property(p_instance->owner, *index, *dst);
 
-#ifdef DEBUG_ENABLED
+				bool ok = ClassDB::get_property(p_instance->owner, *index, *dst);
 				if (!ok) {
 					err_text = "Internal error getting property: " + String(*index);
 					OPCODE_BREAK;
@@ -752,13 +749,13 @@ Variant GDScriptFunction::call(GDScriptInstance *p_instance, const Variant **p_a
 			OPCODE(OPCODE_ASSIGN_TYPED_BUILTIN) {
 
 				CHECK_SPACE(4);
-				Variant::Type var_type = (Variant::Type)_code_ptr[ip + 1];
 				GET_VARIANT_PTR(dst, 2);
 				GET_VARIANT_PTR(src, 3);
 
+#ifdef DEBUG_ENABLED
+				Variant::Type var_type = (Variant::Type)_code_ptr[ip + 1];
 				GD_ERR_BREAK(var_type < 0 || var_type >= Variant::VARIANT_MAX);
 
-#ifdef DEBUG_ENABLED
 				if (src->get_type() != var_type) {
 					if (Variant::can_convert_strict(src->get_type(), var_type)) {
 						Variant::CallError ce;
@@ -1282,10 +1279,11 @@ Variant GDScriptFunction::call(GDScriptInstance *p_instance, const Variant **p_a
 						OPCODE_BREAK;
 					}
 #endif
+
 					Object *obj = argobj->operator Object *();
 					String signal = argname->operator String();
-#ifdef DEBUG_ENABLED
 
+#ifdef DEBUG_ENABLED
 					if (!obj) {
 						err_text = "First argument of yield() is null.";
 						OPCODE_BREAK;
@@ -1302,13 +1300,13 @@ Variant GDScriptFunction::call(GDScriptInstance *p_instance, const Variant **p_a
 						OPCODE_BREAK;
 					}
 
-#endif
 					Error err = obj->connect(signal, gdfs.ptr(), "_signal_callback", varray(gdfs), Object::CONNECT_ONESHOT);
-#ifdef DEBUG_ENABLED
 					if (err != OK) {
 						err_text = "Error connecting to signal: " + signal + " during yield().";
 						OPCODE_BREAK;
 					}
+#else
+					obj->connect(signal, gdfs.ptr(), "_signal_callback", varray(gdfs), Object::CONNECT_ONESHOT);
 #endif
 				}
 
diff --git a/modules/gdscript/gdscript_parser.cpp b/modules/gdscript/gdscript_parser.cpp
index 2fa5084d84..a79fcccaeb 100644
--- a/modules/gdscript/gdscript_parser.cpp
+++ b/modules/gdscript/gdscript_parser.cpp
@@ -56,7 +56,9 @@ T *GDScriptParser::alloc_node() {
 	return t;
 }
 
+#ifdef DEBUG_ENABLED
 static String _find_function_name(const GDScriptParser::OperatorNode *p_call);
+#endif // DEBUG_ENABLED
 
 bool GDScriptParser::_end_statement() {
 
@@ -747,7 +749,6 @@ GDScriptParser::Node *GDScriptParser::_parse_expression(Node *p_parent, bool p_s
 			while (!bfn && b) {
 				if (b->variables.has(identifier)) {
 					IdentifierNode *id = alloc_node<IdentifierNode>();
-					LocalVarNode *lv = b->variables[identifier];
 					id->name = identifier;
 					id->declared_block = b;
 					id->line = id_line;
@@ -755,6 +756,7 @@ GDScriptParser::Node *GDScriptParser::_parse_expression(Node *p_parent, bool p_s
 					bfn = true;
 
 #ifdef DEBUG_ENABLED
+					LocalVarNode *lv = b->variables[identifier];
 					switch (tokenizer->get_token()) {
 						case GDScriptTokenizer::TK_OP_ASSIGN_ADD:
 						case GDScriptTokenizer::TK_OP_ASSIGN_BIT_AND: