RFR: 8377044: Shenandoah: Convert ShenandoahHeap related code to use Atomic<T>

[William Kemper]

On Wed, 11 Feb 2026 18:12:58 GMT, Ben Taylor <btaylor at openjdk.org> wrote:

> Modernizes the code as has been done in other areas of Shenandoah and G1 GCs.
> 
> The same tier1 tests pass before and after this change on x86_64 and aarch64 linux when run with `-XX:+UseShenandoahGC`

Marked as reviewed by wkemper (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/29676#pullrequestreview-3793268590