Register merges participate in the same parallel gap move as phi inputs,
but their allocation is not aware of the phis' existence (since the
register merge allocation sees the register state _before_ phi input
allocation, which is because that's what parallel move requires). This
means that they might move into a register that is used by a Phi, and
possibly will clobber its value.

Avoid this by recording what registers phis move values into during code
gen, and skipping register moves into those registers. Also DCHECK that
the recorded gap moves can't clobber a target register from a previous
gap move. Additionally, add printing for register merges (both in
regalloc tracing and graph printing).

Bug: v8:7700
Change-Id: I8bd4803a30a894c5654e33fc5657ef3fe6cf7a0b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3791062Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Victor Gomes <victorgomes@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82036}