RFR: 8338336: GenShen: Cleanup stale TODOs [v2]

[William Kemper]

> First batch of stale TODO cleanup. Code changes here are to complete `TODOs` to hoist affiliation check out of closures in full gc and to use `heap locked or safepoint` assert everywhere (which does not require executing thread to be the VMThread).

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains eight additional commits since the last revision:

 - Merge remote-tracking branch 'shenandoah/master' into fix-stale-todos
 - Implement TODO to use an API for excluding free regions
 - Remove or rephrase confusing TODO items
 - Remove out-dated TODOs
 - Remove invalid TODO and vestige of IU mode
 - No, we must use a mask to test for a set bit
 - Soften assertion that heap is locked or we're on a safepoint
 - Heap usage accounting should be working now

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/469/files
  - new: https://git.openjdk.org/shenandoah/pull/469/files/3b368834..40fca6e5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=469&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=469&range=00-01

  Stats: 46 lines in 14 files changed: 9 ins; 30 del; 7 mod
  Patch: https://git.openjdk.org/shenandoah/pull/469.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/469/head:pull/469

PR: https://git.openjdk.org/shenandoah/pull/469