[llvm] [AArch64] use `isTRNMask` to calculate shuffle costs (PR #171524)

[Benjamin Maxwell via llvm-commits]

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@@ -6113,7 +6113,8 @@ AArch64TTIImpl::getShuffleCost(TTI::ShuffleKind Kind, VectorType *DstTy,
   unsigned Unused;
   if (LT.second.isFixedLengthVector() &&
       LT.second.getVectorNumElements() == Mask.size() &&
-      (Kind == TTI::SK_PermuteTwoSrc || Kind == TTI::SK_PermuteSingleSrc) &&
+      (Kind == TTI::SK_PermuteTwoSrc || Kind == TTI::SK_PermuteSingleSrc ||
+       Kind == TTI::SK_InsertSubvector) &&
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MacDue wrote:

Maybe leave a comment about why `SK_InsertSubvector` is included here?

https://github.com/llvm/llvm-project/pull/171524