RFR: Cleanups, TODOs, asserts (part 2)

Tue Apr 11 15:06:19 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:
 - [ ] macos-aarch64-server-fastdebug (some pre-existing failures)
 - [ ] GHA builds

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Commit messages:
 - More cleanup
 - More work
 - More work
 - More work
 - Work

Changes: https://git.openjdk.org/shenandoah/pull/252/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=252&range=00
  Stats: 333 lines in 21 files changed: 81 ins; 72 del; 180 mod
  Patch: https://git.openjdk.org/shenandoah/pull/252.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/252/head:pull/252

PR: https://git.openjdk.org/shenandoah/pull/252