Integrated: 8332913: GenShen: Restore shared update refs iterator

[William Kemper]

Clean backport.

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Commit messages:
 - Backport 31bca3076634a3119a049283587dedd1c3de9f16

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/53/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=53&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332913
  Stats: 21 lines in 4 files changed: 11 ins; 3 del; 7 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/53.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/53/head:pull/53

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/53