RFR: Cleanups, TODOs, asserts (part 1) [v5]

[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 with a new target base due to a merge or a rebase. The pull request now contains 15 commits:

 - Merge branch 'master' into upstream-cleanup-1
 - Refine TODO
 - Remove unnecessary comment
 - Fix a little indentation error
 - New assert fires, but instead of fixing it, remove useless live_memory completely
 - Reverse bad fix
 - More fixes
 - More work
 - More improvements
 - More touchups
 - ... and 5 more: https://git.openjdk.org/shenandoah/compare/461f0831...c0af0d7e

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

Changes: https://git.openjdk.org/shenandoah/pull/245/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=245&range=04
  Stats: 529 lines in 42 files changed: 143 ins; 81 del; 305 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