[turbofan] Also constant-fold Object.getPrototypeOf if possible.

test/mjsunit/compiler/object-getprototypeof.js

Index: test/mjsunit/compiler/object-getprototypeof.js
diff --git a/test/mjsunit/compiler/object-getprototypeof.js b/test/mjsunit/compiler/object-getprototypeof.js
new file mode 100644
index 0000000000000000000000000000000000000000..ac172dbeb2557ee1dabd218a4a7e27bd02c1082a
--- /dev/null
+++ b/test/mjsunit/compiler/object-getprototypeof.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
+
+var prototype = Object.create(null);
+var object = Object.create(prototype);
+
+function foo() { return Object.getPrototypeOf(object); }
+
+assertSame(prototype, foo());
+assertSame(prototype, foo());
+%OptimizeFunctionOnNextCall(foo);
+assertSame(prototype, foo());