gc: fix &^=

Fixes #1394.

R=ken2
CC=golang-dev
https://golang.org/cl/3879041

src/cmd/gc/walk.c

@@ -960,15 +960,15 @@ walkexpr(Node **np, NodeList **init)
 		goto ret;
 
 	case OASOP:
-		n->left = safeexpr(n->left, init);
-		walkexpr(&n->left, init);
-		l = n->left;
-		walkexpr(&n->right, init);
 		if(n->etype == OANDNOT) {
 			n->etype = OAND;
 			n->right = nod(OCOM, n->right, N);
 			typecheck(&n->right, Erv);
 		}
+		n->left = safeexpr(n->left, init);
+		walkexpr(&n->left, init);
+		l = n->left;
+		walkexpr(&n->right, init);
 
 		/*
 		 * on 32-bit arch, rewrite 64-bit ops into l = l op r.

test/fixedbugs/bug317.go

+// $G $D/$F.go && $L $F.$A && ./$A.out || echo BUG: bug317
+
+// 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.
+
+package main
+
+func main() {
+	x := []uint{0}
+	x[0] &^= f()
+}
+
+func f() uint {
+	return 1<<31 // doesn't panic with 1<<31 - 1
+}