The register that holds the {new_value} for the AtomicCompareExchange8U
has to be a byte register on ia32. There was code to guarantee that, but
after that code there was code that frees the {eax} register, and that
code moved the {new_value} to a different register again. With this CL
we first free {eax}, and then find a byte register for the {new_value}.

R=clemensb@chromium.org

Bug: chromium:1140549
Change-Id: I1679f3f9ab26c5416ea251c7925366ff43336d85
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2491031Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70721}