Bytecode liveness needs a mapping from offset to liveness. This was
previously a hashmap with a very weak hash (the identity function) and
both inserts and lookups showed up as a non-trivial costs during
compilation.

Now, replace the hashmap with a simple flat array of liveness, indexed
by offset, pre-sized to the size of the bytecode. This will have a lot
of empty entries, but will have much better runtime performance and
probably ends up not much less memory efficient as a hashmap if the
hashmap has to resize inside the Zone, and is likely negligible compared
to the other compilation memory overheads.

Change-Id: Id21375bfcbf0d53b5ed9c41f30cdf7fde66ee699
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3455802Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79049}