RFR: 8377704: Shenandoah: Convert ShenandoahNMethod to use Atomic<T>

[Aleksey Shipilev]

On Thu, 12 Feb 2026 16:32:25 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

Looks fine, thanks.

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Marked as reviewed by shade (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/29698#pullrequestreview-3792415838