In my previous change I mixed up "compare" and "equals". This made us
miss the fast length check before comparing strings for equality.

Now we have a separate helper for "equals". It shares some of the
inner loop details with "compare". I'll see if this can be cleaned up
without making it unreadable.

Review URL: http://codereview.chromium.org/6928020

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7794 ce2b1a6d-e550-0410-aec6-3dcde31c8c00