MIPS: Fix gbemu performance regression.

Port r13557 (aa3ba8a)

BUG=

Review URL: https://codereview.chromium.org/12091083
Patch from Paul Lind <plind44@gmail.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13567 ce2b1a6d-e550-0410-aec6-3dcde31c8c00