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

[William Kemper]

Clean backport.

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Commit messages:
 - Backport 278d3c9adc0882ec9d2f582a5e18263b6bf83b77

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/133/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=133&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-jdk21u/pull/133.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/133/head:pull/133

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/133