[llvm] [NVPTX] Eliminate `prmt`s that result from `BUILD_VECTOR` of `LoadV2` (PR #149581)

[Justin Fargnoli via llvm-commits]

justinfargnoli wrote:

@AlexMaclean suggested extending this to `NVPTXISD::LoadV*`. I'd love to do this, but I wasn't able to create a test case that would be impacted. If anyone's able to make one, please let me know! 

https://github.com/llvm/llvm-project/pull/149581