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

[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.

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Commit messages:
 - Merge remote-tracking branch 'shenandoah/master' into coalesce-old-pinned-regions
 - Allow pinned/regular regions into mixed collection candidates
 - Little clean up

Changes: https://git.openjdk.org/shenandoah/pull/420/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=420&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329790
  Stats: 60 lines in 3 files changed: 22 ins; 17 del; 21 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