MIPS: Use the same registers for StoreIC and KeyedStoreIC.

Port r22387 (ff76c666)

BUG=
R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/390263002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22397 ce2b1a6d-e550-0410-aec6-3dcde31c8c00