Convert more uses of index into offsets. We record spill in terms of
offsets (bytes) rather than slot index, so the name of the method can be
changed, and in GetTotalFrameSlotCount we calculate the number of slots
used in terms of number of bytes spilled.

Bug: v8:9909
Change-Id: I26484c1b040cd4711cc7998cb29d68955bf8ddb6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1934528Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65284}