[GDScript] Perform update-and-assign operations in place when possible.

This turns two bytecode operations into one by using the assignment
destination directly as the output of the binary operator. This manifests
in operations like `+=`.

modules/gdscript/gdscript_compiler.cpp

@@ -1171,8 +1171,18 @@ GDScriptCodeGenerator::Address GDScriptCompiler::_parse_expression(CodeGen &code
 				bool has_operation = assignment->operation != GDScriptParser::AssignmentNode::OP_NONE;
 				if (has_operation) {
 					// Perform operation.
-					GDScriptCodeGenerator::Address op_result = codegen.add_temporary(_gdtype_from_datatype(assignment->get_datatype(), codegen.script));
 					GDScriptCodeGenerator::Address og_value = _parse_expression(codegen, r_error, assignment->assignee);
+
+					if (!has_setter && !assignment->use_conversion_assign) {
+						// If there's nothing special about the assignment, perform the assignment as part of the operator
+						gen->write_binary_operator(target, assignment->variant_op, og_value, assigned_value);
+						if (assigned_value.mode == GDScriptCodeGenerator::Address::TEMPORARY) {
+							gen->pop_temporary(); // Pop assigned value if not done before.
+						}
+						return GDScriptCodeGenerator::Address();
+					}
+
+					GDScriptCodeGenerator::Address op_result = codegen.add_temporary(_gdtype_from_datatype(assignment->get_datatype(), codegen.script));
 					gen->write_binary_operator(op_result, assignment->variant_op, og_value, assigned_value);
 					to_assign = op_result;