RFR: 8338477: GenShen: Cleanup generational heap [v2]

[William Kemper]

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

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:

 - Merge remote-tracking branch 'shenandoah/master' into gen-heap-feedback
 - Inline heap accessor, break up huge do_work method

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Changes: https://git.openjdk.org/shenandoah/pull/474/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=474&range=01
  Stats: 221 lines in 2 files changed: 87 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