RFR: 8377126: Shenandoah: Convert ShenandoahVerifier related code to use Atomic<T> [v2]

[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`.

Ben Taylor has updated the pull request incrementally with one additional commit since the last revision:

  Restore dropped  in shenandoahVerifier

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Changes:
  - all: https://git.openjdk.org/jdk/pull/29576/files
  - new: https://git.openjdk.org/jdk/pull/29576/files/f5c311b4..87229bad

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29576&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29576&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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