[turbofan] Fix buggy implicit coercion in GetMapWitness.

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

Index: test/mjsunit/regress/regress-crbug-715151.js
diff --git a/test/mjsunit/regress/regress-crbug-715151.js b/test/mjsunit/regress/regress-crbug-715151.js
new file mode 100644
index 0000000000000000000000000000000000000000..c0b2c5dba8cba14b976ea7755d0c4a0728a74528
--- /dev/null
+++ b/test/mjsunit/regress/regress-crbug-715151.js
@@ -0,0 +1,15 @@
+// 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();