As discussed offline, the current implementation implement each
situation separately. I think we can simplify the code a lot by sharing
code between the different paths.
This CL does that by
1) implementing the kI64Store case separately, because it does not have
   all the register contraints that the others have, and
2) moving all logic to ensure that the {src} register is usable before
   the switch, such that it's shared by all the compare-exchange cases.

As a side produce, this also fixes issue 1045225, because for i64 stores
which actually only use the lower half of {src}, only that half will be
pinned.

R=ahaas@chromium.org

Bug: chromium:1045225, v8:10108
Change-Id: I0be025b9706d563835ae6337d45b88e0233eacad
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2029414Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66062}