MIPS: Introduce FeedbackVectorSlot type - better than int.

Port r24528 (f7b7e5a)

Original commit message:
It's good to have typing around this value.

BUG=
R=paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/644583004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24534 ce2b1a6d-e550-0410-aec6-3dcde31c8c00