never have to convert an ASCII string to UC16 for Irregexp.
* Generate slightly different code when we know the subject string
  is ASCII.
Review URL: http://codereview.chromium.org/13247

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@941 ce2b1a6d-e550-0410-aec6-3dcde31c8c00