[turbofan] Properly guard keyed stores wrt. setters in the prototype chain.

test/mjsunit/regress/regress-5275-2.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(x) {
-  return x instanceof Proxy;
+  var a = new Array(1);
+  a[0] = x;
+  return a;
 }
 
-assertFalse(foo({}));
-assertFalse(foo({}));
+assertEquals([1], foo(1));
+assertEquals([1], foo(1));
 %OptimizeFunctionOnNextCall(foo);
-assertFalse(foo({}));
+assertEquals([1], foo(1));
+Object.prototype.__defineSetter__("0", function() {});
+assertEquals([undefined], foo(1));