[crankshaft] Properly deal with null prototype.

test/mjsunit/regress/regress-crbug-679202.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 x = Object.prototype;
+
+function f() { return x <= x; }
+
+f();
+f();
+%OptimizeFunctionOnNextCall(f);
+f();