[llvm] [SelectionDAG] Add PARTIAL_REDUCE_U/SMLA ISD Nodes (PR #125207)

[Sander de Smalen via llvm-commits]

https://github.com/sdesmalen-arm commented:

Thanks, this is looking pretty good! Just a few more minor comments before I'm happy to accept it.

https://github.com/llvm/llvm-project/pull/125207