RFR: 8338528: GenShen: Cleanup shenandoahHeapRegion

[William Kemper]

* Break up huge methods in shenandoahHeapRegion
* Clarify comments
* Change confusing method names

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Commit messages:
 - Merge remote-tracking branch 'shenandoah/master' into cleanup-heap-region
 - Break up large method, rename confusing method, fix typos, improve comments

Changes: https://git.openjdk.org/shenandoah/pull/476/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=476&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338528
  Stats: 79 lines in 6 files changed: 29 ins; 28 del; 22 mod
  Patch: https://git.openjdk.org/shenandoah/pull/476.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/476/head:pull/476

PR: https://git.openjdk.org/shenandoah/pull/476