RFR: 8338528: GenShen: Cleanup shenandoahHeapRegion [v2]

[William Kemper]

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

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Simplify case for scanning all references within humongous object

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/476/files
  - new: https://git.openjdk.org/shenandoah/pull/476/files/3eca1a04..79f6ca25

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=476&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=476&range=00-01

  Stats: 5 lines in 2 files changed: 3 ins; 1 del; 1 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