[llvm] [ValueTracking][X86] Compute KnownBits for phadd/phsub (PR #92429)

[via llvm-commits]

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@@ -541,6 +541,33 @@ void llvm::processShuffleMasks(
   }
 }
 
+void llvm::getHorizDemandedEltsForFirstOperand(unsigned VectorBitWidth,
+                                               const APInt &DemandedElts,
+                                               APInt &DemandedLHS,
+                                               APInt &DemandedRHS) {
+  int NumLanes = std::max<int>(1, VectorBitWidth / 128);
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mskamp wrote:

It seems that KnownBits computation is very limited for `x86_mmx`. Hence, I think, it's not worth the trouble to keep the computation for the MMX variants of `phadd`. Therefore, I've removed the support and added the suggested assert.

https://github.com/llvm/llvm-project/pull/92429