Added back some utf8 optimizations

R=yangguo@chromium.org
BUG=https://code.google.com/p/v8/issues/detail?id=2551

Review URL: https://codereview.chromium.org/12390057

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13842 ce2b1a6d-e550-0410-aec6-3dcde31c8c00