In AtomicOp64 ClearRegister is called twice to clear the registers r8
and r9. Thereby new registers may get allocated. We forgot to add the
newly allocated registers to pinned after the first call to
ClearRegister, which caused the same registers to be allocated again in
the second ClearRegister, and thereby caused the bug.

R=clemensb@chromium.org

Change-Id: I0d069aea4c9438fe30c30c22406b4075ddf3e95c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2170088
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67445}