Use xrange for common "for i in range(...)" use case

Make the parser eliminate a wasteful allocation and initialisation
of a possibly large array.

modules/gdscript/gd_parser.cpp

@@ -1779,6 +1779,20 @@ void GDParser::_parse_block(BlockNode *p_block,bool p_static) {
 					return;
 				}
 
+				// Little optimisation for common usage "for i in range(...):":
+				// don't create and initialize a possibly huge array as range()
+				// would do, but instead create an iterator using xrange()
+				if (container->type == Node::TYPE_OPERATOR) {
+					OperatorNode *op = static_cast<OperatorNode *>(container);
+					if (op->arguments.size() > 0 &&
+							op->arguments[0]->type == Node::TYPE_BUILT_IN_FUNCTION) {
+						BuiltInFunctionNode *c = static_cast<BuiltInFunctionNode *>(op->arguments[0]);
+						if (c->function == GDFunctions::GEN_RANGE) {
+							c->function = GDFunctions::GEN_XRANGE;
+						}
+					}
+				}
+
 				ControlFlowNode *cf_for = alloc_node<ControlFlowNode>();
 
 				cf_for->cf_type=ControlFlowNode::CF_FOR;