MIPS: Small refactoring of r14058.

BUG=

Review URL: https://codereview.chromium.org/12703030
Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14084 ce2b1a6d-e550-0410-aec6-3dcde31c8c00