[turbofan] Fix buggy implicit coercion in GetMapWitness.

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();