RFR: 8349766: GenShen: Bad progress after degen does not always need full gc

[William Kemper]

Clean backport

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Commit messages:
 - Backport 3ae80bfb6085e1a6bcb551c7b0be8f27b6f9fde9

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/169/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=169&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349766
  Stats: 20 lines in 2 files changed: 17 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/169.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/169/head:pull/169

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/169