[llvm] [X86] optimize ssse3 horizontal saturating add/sub (PR #169591)

[Simon Pilgrim via llvm-commits]

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RKSimon wrote:

You probably just need add the hsat opcodes to the "isHoriz" check in canonicalizeShuffleMaskWithHorizOp?

https://github.com/llvm/llvm-project/pull/169591