Change FeedbackVector into a Torque-generated class, simplifying its
C++ implementation. As a drive-by, also get rid of the lower-cased,
integer-indexed slot accessors, and replace the calls that needed this
integer index with FeedbackSlot (potentially with a new WithOffset
adjustment for extra data slots).

Change-Id: Ie4f7e99b462ef95419edab46b4ac7ceecae9a05f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2398638
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69749}