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

test/mjsunit/compiler/reflect-getprototypeof.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
+
+var prototype = Object.create(null);
+var object = Object.create(prototype);
+
+function foo() { return Reflect.getPrototypeOf(object); }
+
+assertSame(prototype, foo());
+assertSame(prototype, foo());
+%OptimizeFunctionOnNextCall(foo);
+assertSame(prototype, foo());