MIPS: Use a register spec for StoreIC and KeyedStoreIC.

Port r22328 (92275b7a)

Original commit message:
This continues refactoring already applied for LoadIC in r22035 (https://code.google.com/p/v8/source/detail?r=22035).

BUG=
R=gergely@homejinni.com

Review URL: https://codereview.chromium.org/383913002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22336 ce2b1a6d-e550-0410-aec6-3dcde31c8c00