[llvm] [X86][PartialReduction] Lower zext-byte add reductions to vpsadbw (PR #201076)

[Simon Pilgrim via llvm-commits]

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RKSimon wrote:

OK - let's keep it as it is - I'm slowly adding full ISD::VECREDUCE_ADD support to x86 (which is why I've been slow to review this - I'd hoped to get it done first to avoid this chicken-and-egg problem). #205098 was an early attempt that I might revive - see the cleanup in X86PartialReduction.cpp for reference.

https://github.com/llvm/llvm-project/pull/201076