gc: fix string comparisons for new bool rules

The two string comparison optimizations were
missing the implicit cast from ideal bool.

Fixes #3119.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5696071

src/cmd/gc/walk.c

@@ -1017,6 +1017,7 @@ walkexpr(Node **np, NodeList **init)
 			r = nod(n->etype, nod(OLEN, n->left, N), nod(OLEN, n->right, N));
 			typecheck(&r, Erv);
 			walkexpr(&r, init);
+			r->type = n->type;
 			n = r;
 			goto ret;
 		}
@@ -1029,6 +1030,7 @@ walkexpr(Node **np, NodeList **init)
 			r = nod(n->etype, nod(OLEN, n->left->left, N), nodintconst(0));
 			typecheck(&r, Erv);
 			walkexpr(&r, init);
+			r->type = n->type;
 			n = r;
 			goto ret;
 		}

test/fixedbugs/bug425.go

+// compile
+
+// Copyright 2012 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.
+
+// http://code.google.com/p/go/issues/detail?id=3119
+
+package main
+
+import "fmt"
+
+func main() {
+	s := "hello"
+	fmt.Println(s == "")
+	fmt.Println(s + "world" == "world")
+}