{AtomicSub} on x64 first negates the {value} register, then does an
atomic addition. For that reason, {value} should be a unique register.
So far, we only checked that it's not used in the value stack, but we
should also check for overlap with the destination address or the offset
register.

Drive-by: Remove unneeded handling of non-unique register index on arm,
as that cannot happen (LiftoffCompiler ensures that the result register
is unique).

R=thibaudm@chromium.org

Bug: chromium:1296876
Change-Id: Ie6b97eec8e8dea07b0bcc644d261f47467cc5b8e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3487987Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79265}