Fix wrong counting of function argument usage

There's no need to subtract 1 from the assignment usages because it's
not incremented anywhere else.

Also put back the assignment with operators because they should not
count as usage if the argument is on the left side.

modules/gdscript/gdscript_parser.cpp

@@ -858,11 +858,23 @@ GDScriptParser::Node *GDScriptParser::_parse_expression(Node *p_parent, bool p_s
 				if (current_function) {
 					int arg_idx = current_function->arguments.find(identifier);
 					if (arg_idx != -1) {
-						if (tokenizer->get_token() == GDScriptTokenizer::TK_OP_ASSIGN) {
-							// Assignment is not really usage
-							current_function->arguments_usage.write[arg_idx] = current_function->arguments_usage[arg_idx] - 1;
-						} else {
-							current_function->arguments_usage.write[arg_idx] = current_function->arguments_usage[arg_idx] + 1;
+						switch (tokenizer->get_token()) {
+							case GDScriptTokenizer::TK_OP_ASSIGN_ADD:
+							case GDScriptTokenizer::TK_OP_ASSIGN_BIT_AND:
+							case GDScriptTokenizer::TK_OP_ASSIGN_BIT_OR:
+							case GDScriptTokenizer::TK_OP_ASSIGN_BIT_XOR:
+							case GDScriptTokenizer::TK_OP_ASSIGN_DIV:
+							case GDScriptTokenizer::TK_OP_ASSIGN_MOD:
+							case GDScriptTokenizer::TK_OP_ASSIGN_MUL:
+							case GDScriptTokenizer::TK_OP_ASSIGN_SHIFT_LEFT:
+							case GDScriptTokenizer::TK_OP_ASSIGN_SHIFT_RIGHT:
+							case GDScriptTokenizer::TK_OP_ASSIGN_SUB:
+							case GDScriptTokenizer::TK_OP_ASSIGN: {
+								// Assignment is not really usage
+							} break;
+							default: {
+								current_function->arguments_usage.write[arg_idx] = current_function->arguments_usage[arg_idx] + 1;
+							}
 						}
 					}
 				}