Add some more code comments for code snippets that are not obvious,
especially if debug code is enabled.
The comments help when looking at Liftoff code for debugging code
generation issues.

R=thibaudm@chromium.org

Change-Id: I566bf2b05a454fb8addc030359969d36cb2cb707
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2108557Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66791}