Matching a back-reference must handle unbound start-register (but can assume that if start register is bound, then end register is bound too).

After matching a back reference, the character position is advanced past
the match


git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@908 ce2b1a6d-e550-0410-aec6-3dcde31c8c00