Correctify instanceof and make it optimizable.

test/mjsunit/regress/regress-4376-1.js

Index: test/mjsunit/regress/regress-4376-1.js
diff --git a/test/mjsunit/regress/regress-4376-1.js b/test/mjsunit/regress/regress-4376-1.js
new file mode 100644
index 0000000000000000000000000000000000000000..edb97ee2a9abe25ff012a665ed72f21e6e717fc3
--- /dev/null
+++ b/test/mjsunit/regress/regress-4376-1.js
@@ -0,0 +1,12 @@
+// Copyright 2015 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.
+
+function Bar() { }
+function Baz() { }
+Baz.prototype = { __proto__: new Bar() }
+var x = new Baz();
+function foo(y) { return y instanceof Bar; }
+assertTrue(foo(x));
+Baz.prototype.__proto__ = null;
+assertFalse(foo(x));