Handlify RegExpKey.

This fixed mjsunit/unicode-case-overoptimization after r20578.

BUG=
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/228483004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20579 ce2b1a6d-e550-0410-aec6-3dcde31c8c00