[llvm] [NVPTX] Only run LowerUnreachable when necessary (PR #109868)

[Justin Fargnoli via llvm-commits]

justinfargnoli wrote:

> `trap`, AFAICT, is the best way to do so that we have right now.

I followed-up internally and confirmed that `trap` is the best option that we have right now. 

>  if we do, or on the PTX (or ptxas) version.

I've confirmed that this was fixed in CUDA 12.3 which corresponds with PTX version 8.3.

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> whether we want/need to emit the unreachable hint instruction to PTX

Assuming that we do want to emit the unreachable hint (I don't think anyone has advocated against this), then this PR should be ready for another round of review. 

https://github.com/llvm/llvm-project/pull/109868