A mix of readability refactors, additional DCHECKs, and
addressed/updated TODOs.

Bug: v8:7790
Change-Id: I87ff996abd40b0ed081586e2c0da1a4c0942fed4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3041665
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75858}