[llvm] [NVPTX] Cleanup NVPTXLowerArgs, simplifying logic and improving alignment propagation (PR #180286)

[Alex MacLean via llvm-commits]

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AlexMaclean wrote:

I've remove this logic entirely, we will now never process conditionals such as Phis or Selects in this function. 

https://github.com/llvm/llvm-project/pull/180286