RFR: 8342861: GenShen: Old generation in unexpected state when abandoning mixed gc candidates

[William Kemper]

When the generational mode cannot add any old regions to a mixed collection because they are _all_ pinned, it gives up trying to evacuate these regions. The regions must then be made parsable. However, if this phase is interrupted by a young collection it should no longer try to evacuate any old regions. Doing so triggered an assert when testing with `-XX:ShenandoahCoalesceChance=80` and `XX:+ShenandoahAllocFailureALot`.

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Commit messages:
 - Do not add old regions to collection set once filling has started

Changes: https://git.openjdk.org/shenandoah/pull/522/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=522&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342861
  Stats: 10 lines in 2 files changed: 10 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/522.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/522/head:pull/522

PR: https://git.openjdk.org/shenandoah/pull/522