X87: Introduce FeedbackVectorSlot type - better than int.

port r24528.

original commit message:

  Introduce FeedbackVectorSlot type - better than int.

  It's good to have typing around this value.

BUG=
R=weiliang.lin@intel.com

Review URL: https://codereview.chromium.org/645323002

Patch from Chunyang Dai <chunyang.dai@intel.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24540 ce2b1a6d-e550-0410-aec6-3dcde31c8c00