RFR: Cleanups, TODOs, asserts (part 1) [v4]

[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)

Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:

  Refine TODO

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

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/245/files
  - new: https://git.openjdk.org/shenandoah/pull/245/files/57f99c66..a6ca5046

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=245&range=03
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=245&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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