[llvm] [NVPTX] Generalize and extend upsizing when lowering 8/16-bit-element vector loads/stores (PR #119622)

[Alex MacLean via llvm-commits]

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

LGTM. merging on @dakersnar's behalf per his offline request. 

https://github.com/llvm/llvm-project/pull/119622