RFR: 8338477: GenShen: Cleanup generational heap

[William Kemper]

* Inline heap accessor
* break up huge do_work method
* remove TODOs

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Commit messages:
 - Inline heap accessor, break up huge do_work method

Changes: https://git.openjdk.org/shenandoah/pull/474/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=474&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338477
  Stats: 224 lines in 2 files changed: 90 ins; 95 del; 39 mod
  Patch: https://git.openjdk.org/shenandoah/pull/474.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/474/head:pull/474

PR: https://git.openjdk.org/shenandoah/pull/474