Turn RegExpConstructResultStub into a HydrogenCodeStub.

This has the additional benefit that it is now possible to
inline the RegExpResult construction code into Hydrogen
builtins.

R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/141703018

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18902 ce2b1a6d-e550-0410-aec6-3dcde31c8c00