Recompute the spill offsets for values in the merge region, instead of
reusing the offsets of the source. This ensures that spill slots stay
contiguous (modulo alignment).
This also solves a correctness issue where the spill offsets in the
merge region could move up, thereby overwriting the source of another
move.
With this change, the spill offsets always move down (to fill the gap)
or stay the same, such that processing them from bottom to top
can only overwrite sources of already-processed moves.

Since we do not reuse the current state's offsets, this might generate
extra stack moves and regress generated code performance a bit.

Drive-by: print spill offsets in the Liftoff trace

R=clemensb@chromium.org

Bug: v8:12270
Change-Id: I8d20df8fc1e80dd36b6f651de457686e9935a628
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3245115
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77556}