[crankshaft] Fix deopt loop in String.fromCharCode on non-int32 inputs.

test/mjsunit/regress/regress-crbug-587068.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(a) {
-  for (var d in a) {
-    delete a[1];
-  }
-}
-
-foo([1,2,3]);
-foo([2,3,4]);
+// The Crankshaft fast case for String.fromCharCode used to unconditionally
+// deoptimize on non int32 indices.
+function foo(i) { return String.fromCharCode(i); }
+foo(33);
+foo(33);
 %OptimizeFunctionOnNextCall(foo);
-foo([1,2,3]);
+foo(33.3);
 assertOptimized(foo);