[crankshaft] only compile string index access with element key.

test/mjsunit/regress/regress-crbug-554831.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();
-  }
-}
-%OptimizeFunctionOnNextCall(foo);
-foo();
+(function() {
+  var key = "s";
+  function f(object) { return object[key]; };
+  f("");
+  f("");
+  %OptimizeFunctionOnNextCall(f);
+  f("");
+  assertOptimized(f);
+})();