[llvm] [DAG] Change `isExtractSubvectorCheap` into `getExtractSubvectorCost` (PR #213614)

[Luke Lau via llvm-commits]

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@@ -27777,7 +27777,7 @@ static SDValue narrowExtractedVectorBinOp(EVT VT, SDValue Src, unsigned Index,
   // bitcasted.
   unsigned ConcatOpNum = Index / VT.getVectorNumElements();
   unsigned ExtBOIdx = ConcatOpNum * NarrowBVT.getVectorNumElements();
-  if (TLI.isExtractSubvectorCheap(NarrowBVT, WideBVT, ExtBOIdx) &&
+  if ((TLI.getExtractSubvectorCost(NarrowBVT, WideBVT, ExtBOIdx) < 2) &&
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lukel97 wrote:

Comparing by 2 seems pretty arbitrary. I think cheap in most of these targets refers to being free? At least on RISC-V. Can we just check that the cost is `TCC_Free`?

https://github.com/llvm/llvm-project/pull/213614