RFR: 8342861: GenShen: Old generation in unexpected state when abandoning mixed gc candidates [v2]

[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`.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Problem list failing reference processing test

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/522/files
  - new: https://git.openjdk.org/shenandoah/pull/522/files/1356221f..8532d08f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=522&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=522&range=00-01

  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 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