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

[David Sherwood 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,
----------------
david-arm wrote:

Yeah I did wonder that myself. I just wasn't sure if `LastIndex` might also be convenient for callers to avoid calculating the last index for fixed-width vectors each time. I have no problem adding an assert if others agree?

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