This groups atomicops by type, removes an unused atomicop, and fixes
comments.

Change-Id: I9621ac4512d019ae9d0ce4f7e38c81011d46aa37
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2047044Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66201}