- Reduce it to half size if the pattern is ASCII, saving on initialization
- If pattern is ASCII and subject is not, any non-ASCII char can cause a
  full pattern-length shift, even if we haven't indexed the entire pattern.
- Use memset to initialize buffer in the common case where the pattern is
  shorter than the max significant suffix limit.


git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@519 ce2b1a6d-e550-0410-aec6-3dcde31c8c00