[llvm] Port `NVPTXTargetLowering::LowerCONCAT_VECTORS` to SelectionDAG (PR #120030)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

Actual change LGTM, but we probably need some kind of legality heuristic for which legalization path to prefer 

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