[llvm] [LV][TTI] Calculate cost of extracting last index in a scalable vector (PR #144086)

[Luke Lau via llvm-commits]

================
@@ -1465,14 +1465,21 @@ class TargetTransformInfo {
       OperandValueInfo Op2Info = {OK_AnyValue, OP_None},
       const Instruction *I = nullptr) const;
 
+  enum : int {
+    UnknownIndex = -1,
+    LastIndex = -2,
----------------
lukel97 wrote:

It looks like one of the callers does the calculation whilst the other doesn't:

```c++
TTI.getVectorInstrCost(
                  Instruction::ExtractElement, VectorTy, CostKind,
                  VF.isScalable() ? TargetTransformInfo::LastIndex
                                  : VF.getKnownMinValue() - 1));
```

I don't have a strong opinion on this btw, just seems like it might be easier to handle this in the callers since there's fewer of them than target hooks. 

https://github.com/llvm/llvm-project/pull/144086