The mid-tier register allocator could not handle the case that the same
virtual register was used for
- the input corresponding to the 'same-as-input' output, and
- another 'unique register' input.

In this case, it cannot choose the already assigned register for the
'unique' register. Instead, it needs to allocate a new register and
introduce a gap move to duplicate the input value in two different
registers.

FYI, the instruction where the current logic failed was:
  (v5(0), v6(R)) = IA32AddPair v7(R) v7(*) v8(R) v7(R)
(where the last input was marked 'unique').

R=leszeks@chromium.org
CC=thibaudm@chromium.org

Bug: v8:12330, chromium:1272204
Change-Id: Ie4843aa9f5e027afe503e0481a4acdfa325dfe0e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3347821Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78411}