RFR: 8368152: Shenandoah: Incorrect behavior at end of degenerated cycle

[William Kemper]

Not clean, but trivial conflict resolution.

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Commit messages:
 - Add missing precompile header for 21 branch
 - 8368152: Shenandoah: Incorrect behavior at end of degenerated cycle

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/222/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=222&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368152
  Stats: 145 lines in 5 files changed: 112 ins; 19 del; 14 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/222.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/222/head:pull/222

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/222