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

test/mjsunit/regress/regress-crbug-554831.js

Index: test/mjsunit/regress/regress-crbug-554831.js
diff --git a/test/mjsunit/compiler/regress-491578.js b/test/mjsunit/regress/regress-crbug-554831.js
similarity index 54%
copy from test/mjsunit/compiler/regress-491578.js
copy to test/mjsunit/regress/regress-crbug-554831.js
index c27570456c31e9a191fe1f4f1ea17b80f29de98e..f7343e08b90a1a484179907c34d7a469e93aeec1 100644
--- a/test/mjsunit/compiler/regress-491578.js
+++ b/test/mjsunit/regress/regress-crbug-554831.js
@@ -4,12 +4,12 @@
 
 // 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);
+})();