[turbofan] Fix buggy implicit coercion in GetMapWitness.

BUG=chromium:715151
R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/2839873004
Cr-Commit-Position: refs/heads/master@{#44881}

src/compiler/js-builtin-reducer.cc

@@ -113,8 +113,10 @@ MaybeHandle<Map> GetMapWitness(Node* node) {
   ZoneHandleSet<Map> maps;
   Node* receiver = NodeProperties::GetValueInput(node, 1);
   Node* effect = NodeProperties::GetEffectInput(node);
-  if (NodeProperties::InferReceiverMaps(receiver, effect, &maps)) {
-    if (maps.size() == 1) return MaybeHandle<Map>(maps[0]);
+  NodeProperties::InferReceiverMapsResult result =
+      NodeProperties::InferReceiverMaps(receiver, effect, &maps);
+  if (result == NodeProperties::kReliableReceiverMaps && maps.size() == 1) {
+    return maps[0];
   }
   return MaybeHandle<Map>();
 }

test/mjsunit/regress/regress-crbug-715151.js

+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --allow-natives-syntax --verify-heap
+
+function foo() {
+  var a = [0];
+  Object.preventExtensions(a);
+  return a.pop();
+}
+foo();
+foo();
+%OptimizeFunctionOnNextCall(foo);
+foo();