Stick more closely to the decoding guide laid out in the manual, and
also take the chance to remove some duplicate code.

Drive-by fix a clang-tidy warning for bool literal.

Bug: v8:11074
Change-Id: I91aa8db7cd3db30b250e8bfc9bb146c8bb56dcd1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2567530
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71630}