[llvm] [NVPTX] support switch statement with brx.idx (PR #102400)

[Artem Belevich via llvm-commits]

https://github.com/Artem-B approved this pull request.

LGTM.

Does brx.idx buy us a better code on SASS level? or is this mostly cosmetic sugar on PTX level?

https://github.com/llvm/llvm-project/pull/102400