Also clean up some comments in liftoff-assembler-x64.h.

Bug: v8:11589
Change-Id: I47fe5c2c794c863be1afde86d289ea197219a4f8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2787591
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73692}