RFR: Cleanups, TODOs, asserts (part 1)

Wed Apr 5 16:29:45 UTC 2023

I have been reading the `openjdk/shenandoah` vs `openjdk/jdk` diff and these are the simple code massages we can do to improve and bullet-proof the code, hopefully without changing the semantics. This would continue with more parts later.

Additional testing:
 - [x] macos-aarch64-server-fastdebug (some pre-existing failures)

-------------

Commit messages:
 - Reverse bad fix
 - More fixes
 - More work
 - More improvements
 - More touchups
 - More work
 - More work
 - More work
 - Cherry-picks
 - Cleanups

Changes: https://git.openjdk.org/shenandoah/pull/245/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=245&range=00
  Stats: 534 lines in 42 files changed: 148 ins; 77 del; 309 mod
  Patch: https://git.openjdk.org/shenandoah/pull/245.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/245/head:pull/245

PR: https://git.openjdk.org/shenandoah/pull/245