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

[Justin Fargnoli via llvm-commits]

justinfargnoli wrote:

This looks great! 

Check out [this documentation](https://llvm.org/docs/TestingGuide.html#generating-assertions-in-regression-tests) for details on how to fix the test failures. 

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