[llvm] [ISel][AArch64] Add CodeGen support for partial sub reductions. (PR #186809)

[Paul Walker via llvm-commits]

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paulwalker-arm wrote:

Can you add `getPartialMLS()` to SelectionDAG? because the same code exists below and presumably will exist everywhere somebody wants to create this virtual operation.

https://github.com/llvm/llvm-project/pull/186809