Spill/fill now take offsets instead of indices. We provide a
helper, GetStackOffsetFromIndex, for callers. This is currently only
useful while slot sizes are still fixed to 8 bytes.

StackTransferRecipe's RegisterLoad now works in terms of offset.

LiftoffStackSlots work in terms of offset as well.

TransferStackSlot currently still works in terms of indicies, but can be
converted to use offsets in a subsequent change.

Bug: v8:9909
Change-Id: If54fb844309bdfd641720d063135dd59551813e0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1922489Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65049}