Implement tracking and optimizations of packed arrays.

test/mjsunit/regress/regress-1849.js

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 18 matching lines @@
 
 // Flags: --allow-natives-syntax
 
 var count = 1e5;
 var arr = new Array(count);
 assertFalse(%HasFastDoubleElements(arr));
 for (var i = 0; i < count; i++) {
   arr[i] = 0;
 }
 assertFalse(%HasFastDoubleElements(arr));
-assertTrue(%HasFastSmiOnlyElements(arr));
+assertTrue(%HasFastSmiElements(arr));