If a value is expected to be in a particular register for a register
merge, allow for it to be moved there from another register, without
expecting it to be spilled.

Bug: v8:7700
Change-Id: I9ef5e77b3a744a6284f4790ec9d5a7c60739a710
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3793391Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Victor Gomes <victorgomes@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82066}