Previously, the accumulator was at the end of liveness bitvectors, which
meant that checking for accumulator liveness required a length lookup.
This CL moves it to the start of the bitvector, with registers starting
at index 1 -- the assumption is that the addition of 1 to the index on
register liveness access can be constant folded away.

As a cleanup, replace all the custom liveness printing code with a
single unified ToString. This places the accumulator at the end of the
printed liveness, to avoid having to change test expectations (also, the
position of the accumulator is now an implementation detail). As a
similar cleanup, change StateValue node building to use the
BytecodeLivenessState interface rather than the underlying bitvector.
These two cleanups allow us to remove the raw bitvector accessor from
liveness entirely.

Change-Id: Ic2744b5e8e16b8527e6a4e8d3b4ddad7096289d9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3455144
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79066}