RFR: 8377126: Convert ShenandoahVerifier related code to use Atomic<T>

[Ben Taylor]

Modernizes ShenandoahVerifier code by replacing `AtomicAccess` methods with `Atomic<T>` types.

Tier1 test results on x86_64 linux are the same before and after this change when running with `TEST_VM_OPTS=-XX:+UseShenandoahGC`.

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Commit messages:
 - 8377126: Convert ShenandoahVerifier related code to use Atomic<T>

Changes: https://git.openjdk.org/jdk/pull/29576/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29576&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377126
  Stats: 14 lines in 2 files changed: 0 ins; 0 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/29576.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29576/head:pull/29576

PR: https://git.openjdk.org/jdk/pull/29576