Fix stale IC::receiver_map_ after prototype fastification

test/mjsunit/regress/regress-crbug-618845.js

+// Copyright 2016 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
+
+function Foo() {}
+Object.defineProperty(Foo.prototype, "name",
+                      {get: function() { return "FooName"; }});
+
+function ic(f) {
+  return f.prototype.name;
+}
+
+assertEquals("FooName", ic(Foo));
+assertEquals("FooName", ic(Foo));  // Don't crash, don't time out.