RFR: 8375438: G1: Convert G1HeapRegion related classes to use Atomic<T>

[Thomas Schatzl]

Hi all,

  please review conversion of G1HeapRegion related classes to use Atomic<T>.

Testing: tier1, tier4, tier5

(The PipelineLeaksFD failure in gha is a known issue)

Thanks,
  Thomas

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Commit messages:
 - 8375438

Changes: https://git.openjdk.org/jdk/pull/29301/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29301&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375438
  Stats: 55 lines in 8 files changed: 17 ins; 1 del; 37 mod
  Patch: https://git.openjdk.org/jdk/pull/29301.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29301/head:pull/29301

PR: https://git.openjdk.org/jdk/pull/29301