This significantly improves the speed for creating short substrings (less than 13 characters) from slices, flat cons strings and external strings.

TEST=string-external-cached.js, string-slices.js

Review URL: http://codereview.chromium.org/8889012

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10221 ce2b1a6d-e550-0410-aec6-3dcde31c8c00