[llvm] Port NVPTXTargetLowering::LowerCONCAT_VECTORS to llvm/lib/CodeGen/SelectionDAG (PR #120030)

[Justin Fargnoli via llvm-commits]

justinfargnoli wrote:

> I did see this, is it ok to just overwrite test cases like this? I'm worried I may break something I don't understand.

Yes, we will review the diff to ensure it's okay before merging the change! 

https://github.com/llvm/llvm-project/pull/120030