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

[Ethan Kaji via llvm-commits]

================
@@ -2001,28 +2001,6 @@ SDValue NVPTXTargetLowering::LowerSTACKSAVE(SDValue Op,
   return DAG.getMergeValues({ASC, SDValue(SS.getNode(), 1)}, DL);
 }
 
-// By default CONCAT_VECTORS is lowered by ExpandVectorBuildThroughStack()
----------------
Esan5 wrote:

This might be completely wrong, but I think they were removed in 2013 with  be8dc6499ac00b2e5a608dbcec3ecb17348299e0

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