Record the various types of feedback vector slot separately, to estimate
the relative impact of e.g. load ICs vs call ICs. Also, log the unused
(i.e. uninitialized or premonomorphic) ones separately.

Bug: v8:7266
Change-Id: Ie035cf48969e39f7156dfe523fd9218749b95cfe
Reviewed-on: https://chromium-review.googlesource.com/897813
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51067}