Replace two unnecessary compare-exchange operations by simpler fetch_add
or exchange. This makes it easier to read and potentially faster.

R=thibaudm@chromium.org

Change-Id: Id9347ce837863e62887619f905a646bf48c07347
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2377687Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69581}