[llvm] [NVPTX] Fixup some issues introduced by 128-bit atomics (PR #155921)

[Justin Fargnoli via llvm-commits]

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

Change LGTM, but it'd be nice to get a post-commit review from someone that reviewed #154852. 

https://github.com/llvm/llvm-project/pull/155921