RFR: 8329790: GenShen: Old regions that are pinned during final mark are not made parseable [v2]

[William Kemper]

> Pinned regions that are not humongous may be added to the set of _candidates_ for mixed collection. They still may not be added to the collection set for a mixed collection. Regions that are not in this collection of _candidates_ will not be coalesced and filled.

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - Merge remote-tracking branch 'shenandoah/master' into coalesce-old-pinned-regions
 - Use is_regular_pinned to restore candidate selection logic to its previous form
 - Introduce 'regular pinned' state
 - Merge remote-tracking branch 'shenandoah/master' into coalesce-old-pinned-regions
 - Allow pinned/regular regions into mixed collection candidates
 - Little clean up

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/420/files
  - new: https://git.openjdk.org/shenandoah/pull/420/files/b2d5e8ce..b0e3b2ab

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=420&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=420&range=00-01

  Stats: 1918 lines in 37 files changed: 988 ins; 803 del; 127 mod
  Patch: https://git.openjdk.org/shenandoah/pull/420.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/420/head:pull/420

PR: https://git.openjdk.org/shenandoah/pull/420