[turbofan] Add test case for stores to properties that are also present on prototype.

test/mjsunit/compiler/regress-4470-1.js

 // 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.
 
 // Flags: --allow-natives-syntax
 
-function foo(x) {
-  if (x === undefined) return;
-  while (true) {
-    while (1 || 2) { }
-    f();
-  }
+function Foo() {}
+Foo.prototype.x = 0;
+function foo(f) {
+  f.x = 1;
 }
+foo(new Foo);
+foo(new Foo);
 %OptimizeFunctionOnNextCall(foo);
-foo();
+foo(new Foo);
+assertEquals(Foo.prototype.x, 0);