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

[Paul Walker via llvm-commits]

https://github.com/paulwalker-arm approved this pull request.

A few final recommendations but otherwise this looks good to me.  I'll admit to not be convinced about the implementation of `SplitVecRes_PARTIAL_REDUCE_MLA` but figure that'll be short lived and so if you're sure it works then that's good enough for me.

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