Straight-line bytecode with exactly one "next" bytecode (i.e. everything
that can't affect control flow) will always have the same "out" liveness
as the next bytecode's "in" liveness. For those cases, we can save a bit
of time and memory by aliasing the pointers between the bytecode's out
liveness and the next bytecode's in liveness, and skipping copying
between them.

This is done by specializing the current liveness update on whether this
is the first pass (which will allocate and initialize the liveness
bitvectors) or an update pass (which will revisit loops to collect
liveness crossing over the back-edge, and propagate this liveness
through the loop bodies). On the first pass, we can delay allocation of
the out liveness until we know it needs to be union of multiple in
livenesses, and on the update pass we can skip it if it is an alias.

As a drive-by, tweak BitVector::CopyFrom to require copying from a
vector with the same size (same as Union or Intersect), and move the
only different sized vector use (in Resize) to be inline.

Change-Id: Iad1b2e1b927a37ad925ef68e2a224152aaa2ba18
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3350452
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78425}