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

[Simon Pilgrim via llvm-commits]

================
@@ -20309,12 +20309,17 @@ bool AArch64TargetLowering::shouldConvertConstantLoadToIntImm(const APInt &Imm,
   return Shift < 3;
 }
 
-bool AArch64TargetLowering::isExtractSubvectorCheap(EVT ResVT, EVT SrcVT,
-                                                    unsigned Index) const {
+AArch64TargetLowering::ExtractSubvectorCost
----------------
RKSimon wrote:

Shouldn't these be:
```suggestion
TargetLoweringBase::ExtractSubvectorCost
```

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