RFR: Cleanups, TODOs, asserts (part 2) [v2]

[Aleksey Shipilev]

> 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)
>  - [x] GHA builds

Aleksey Shipilev has updated the pull request incrementally with three additional commits since the last revision:

 - More review comments
 - Move and rename remset verifications
 - Review Ramki

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

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/252/files
  - new: https://git.openjdk.org/shenandoah/pull/252/files/906b1cbd..c567172d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=252&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=252&range=00-01

  Stats: 456 lines in 8 files changed: 223 ins; 216 del; 17 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