RFR: 8377043: Shenandoah: Convert ShenandoahHeapRegion related code to use Atomic<T>

[William Kemper]

On Wed, 4 Feb 2026 18:20:17 GMT, Ben Taylor <btaylor at openjdk.org> wrote:

> Modernizes ShenandoahHeapRegion 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`.

Marked as reviewed by wkemper (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/29575#pullrequestreview-3765670882