This was a left-over from a time when bounds-check was performed
as part of this instruction.

I also refactored and improved the code for smi-only arrays.

R=vegorov@chromium.org
Review URL: http://codereview.chromium.org/9023006

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10300 ce2b1a6d-e550-0410-aec6-3dcde31c8c00