RFR: 8323086: Shenandoah: Heap could be corrupted by oom during evacuation

[William Kemper]

When the load reference barrier is unable to evacuate an object, it initiates a degenerated cycle, and allows the from-space reference past the barrier. The degenerated cycle is meant to patch up these references, but if the reference is written above the update-watermark, it will be missed.

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Commit messages:
 - Reset update watermark for cycles degenerated during evacuation

Changes: https://git.openjdk.org/jdk/pull/17285/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17285&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323086
  Stats: 21 lines in 1 file changed: 21 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17285.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17285/head:pull/17285

PR: https://git.openjdk.org/jdk/pull/17285