Introduce FeedbackVectorSlot type - better than int.

It's good to have typing around this value.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/641373002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24528 ce2b1a6d-e550-0410-aec6-3dcde31c8c00