[llvm] [SLPVectorizer, TTI, X86, SystemZ] Move X86 specific handling into X86TTIImpl. (PR #137830)

[Simon Pilgrim via llvm-commits]

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@@ -941,6 +941,7 @@ class TargetTransformInfo {
                                            const APInt &DemandedElts,
                                            bool Insert, bool Extract,
                                            TTI::TargetCostKind CostKind,
+                                           bool ForPoisonSrc = true,
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RKSimon wrote:

It'd be useful if you could include in the doxygen comment what ForPoisonSrc means

https://github.com/llvm/llvm-project/pull/137830