MIPS: Refactor RegExpStub to check lazily.

Port r13727 (55a760ec)

BUG=

Review URL: https://codereview.chromium.org/12374073

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13798 ce2b1a6d-e550-0410-aec6-3dcde31c8c00