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

[Paul Walker via llvm-commits]

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paulwalker-arm wrote:

Perhaps add PatFrags for `partial_reduce_smls` and `partial_reduce_umls` to hide the AArch64ineg stuff?

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