RFR: 8332256: Shenandoah: Do not visit heap threads during shutdown

[William Kemper]

In rare cases, this may lead to a crash or hang during shutdown.

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Commit messages:
 - Do not visit heap threads during shutdown

Changes: https://git.openjdk.org/jdk/pull/19239/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19239&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332256
  Stats: 9 lines in 1 file changed: 7 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/19239.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19239/head:pull/19239

PR: https://git.openjdk.org/jdk/pull/19239