cmd/gc: fix &^ code generation bug

Was not re-walking the new AND node, so that its ullman
count was wrong, so that the code generator attempted to
store values in registers across the call.

Fixes #4752.

R=ken2
CC=golang-dev
https://golang.org/cl/7288054

src/cmd/gc/walk.c

@@ -926,10 +926,10 @@ walkexpr(Node **np, NodeList **init)
 
 	case OANDNOT:
 		walkexpr(&n->left, init);
-		walkexpr(&n->right, init);
 		n->op = OAND;
 		n->right = nod(OCOM, n->right, N);
 		typecheck(&n->right, Erv);
+		walkexpr(&n->right, init);
 		goto ret;
 
 	case ODIV:

test/fixedbugs/issue4752.go

+// run
+
+// Copyright 2013 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 F(xi, yi interface{}) uint64 {
+	x, y := xi.(uint64), yi.(uint64)
+	return x &^ y
+}
+
+func G(xi, yi interface{}) uint64 {
+	return xi.(uint64) &^ yi.(uint64) // generates incorrect code
+}
+
+func main() {
+	var x, y uint64 = 0, 1 << 63
+	f := F(x, y)
+	g := G(x, y)
+	if f != 0 || g != 0 {
+		println("F", f, "G", g)
+		panic("bad")
+	}
+}