RFR: 8348595: GenShen: Fix generational free-memory no-progress check

[William Kemper]

Clean backport

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Commit messages:
 - 8348595: GenShen: Fix generational free-memory no-progress check

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/166/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=166&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348595
  Stats: 27 lines in 5 files changed: 21 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/166.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/166/head:pull/166

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/166