In atomic.notify we overwrote the register which stored the index,
without checking if it was still in use or not.

R=clemensb@chromium.org

Bug: v8:10898
Change-Id: I59ed7a2c1f1342ff4252e3c4d33822111caee82c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2426616Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70125}