gc: fix spurious typecheck loop in &composite literal

Fixes #2538.

R=ken2
CC=golang-dev
https://golang.org/cl/5449114

src/cmd/gc/typecheck.c

@@ -2198,6 +2198,7 @@ typecheckcomplit(Node **np)
 		n->typecheck = 1;
 		n->type = n->left->type;
 		n->left->type = t;
+		n->left->typecheck = 1;
 	}
 
 	*np = n;

test/fixedbugs/bug380.go

+// $G $D/$F.go
+
+// Copyright 2011 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Used to cause a typechecking loop error.
+
+package pkg
+type T map[int]string
+var q = &T{}