Cleanup latin-1 conversion check in regexp engine

R=yangguo@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/11880045
Patch from Dan Carney <dcarney@google.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13400 ce2b1a6d-e550-0410-aec6-3dcde31c8c00