RFR: 8338336: GenShen: Cleanup stale TODOs

[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).

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Commit messages:
 - 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

Changes: https://git.openjdk.org/shenandoah/pull/469/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=469&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338336
  Stats: 114 lines in 12 files changed: 3 ins; 83 del; 28 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