RFR: 8344638: GenShen: Verifier should not touch claim token

[William Kemper]

GenShen is using parallel_threads_do for a verification step, but this uses a 'claim token' which is shared by normal gc activity. The verifier should not touch this shared state, lest it interfere with (or be interfered with by) the gc cycle.

# Testing

Ran this through internal pipelines: Dacapo, Extremem, SpecJBB2015, SpecJVM2008, Heapothesys (with and without GC stress flags).

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Commit messages:
 - Verifier should not touch claim token (lest it unwittingly obscure a defect)

Changes: https://git.openjdk.org/shenandoah/pull/546/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=546&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344638
  Stats: 5 lines in 1 file changed: 0 ins; 4 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/546.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/546/head:pull/546

PR: https://git.openjdk.org/shenandoah/pull/546