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

[Ethan Kaji via llvm-commits]

Esan5 wrote:

@justinfargnoli this is still a work in progress, but would you be willing to look it over to make sure I'm going in the right direction?

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