[llvm] [LoopVectorizer] Add support for partial reductions (PR #92418)

[Graham Hunter via llvm-commits]

================
@@ -341,6 +341,43 @@ class AArch64TTIImpl : public BasicTTIImplBase<AArch64TTIImpl> {
     return BaseT::isLegalNTLoad(DataType, Alignment);
   }
 
+  InstructionCost getPartialReductionCost(unsigned Opcode, Type *InputType,
+                                          Type *AccumType, ElementCount VF,
+                                          PartialReductionExtendKind OpAExtend,
+                                          PartialReductionExtendKind OpBExtend,
+                                          std::optional<unsigned> BinOp) const {
+    InstructionCost Cost = InstructionCost::getInvalid();
----------------
huntergr-arm wrote:

Might be better to name this variable `Invalid` instead of `Cost` to make it clearer that we're not supporting a given case.

https://github.com/llvm/llvm-project/pull/92418