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

[Alex MacLean via llvm-commits]

AlexMaclean wrote:

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

This does lead to improvements on the SASS level in some cases, though this is of course very dependent on `ptxas` optimizations.

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