[llvm] [NVPTX] Fix scoped atomic when given runtime values (PR #185883)

[via llvm-commits]

https://github.com/gonzalobg approved this pull request.

LGTM, thank you Joseph.

Have just left a comment suggestion to capture the intentional lowering of `std::atomic_signal_fence` (before we were just failing, now we are intentionally dropping it, which is fine).

https://github.com/llvm/llvm-project/pull/185883