Fixed parser treating compound assignment the same as assignment which gave wrong argument usage count

modules/gdscript/gdscript_parser.cpp

@@ -776,8 +776,7 @@ GDScriptParser::Node *GDScriptParser::_parse_expression(Node *p_parent, bool p_s
 								}
 								_add_warning(GDScriptWarning::UNASSIGNED_VARIABLE_OP_ASSIGN, -1, identifier.operator String());
 							}
-							FALLTHROUGH;
-						}
+						} break;
 						case GDScriptTokenizer::TK_OP_ASSIGN: {
 							lv->assignments += 1;
 							lv->usages--; // Assignment is not really usage