gc: correct type check for x, ok map assignment

Fixes #288.

R=ken2
https://golang.org/cl/157162

src/cmd/gc/typecheck.c

@@ -2018,7 +2018,7 @@ typecheckas2(Node *n)
 		if(l->type == T)
 			goto out;
 		n->op = OAS2MAPW;
-		n->rlist->n = typecheckconv(nil, r, l->type->down, 0, nil);
+		n->rlist->n = typecheckconv(nil, r, l->type, 0, nil);
 		r = n->rlist->next->n;
 		n->rlist->next->n = typecheckconv(nil, r, types[TBOOL], 0, nil);
 		goto out;

test/fixedbugs/bug220.go

+// $G $D/$F.go || echo BUG: bug220
+
+// Copyright 2009 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.
+
+package main
+
+func main() {
+	m := make(map[int]map[uint]float);
+	
+	m[0] = make(map[uint]float), false;	// 6g used to reject this
+	m[1] = nil;
+}