[llvm] Reland "[LoopVectorizer] Add support for partial reductions" with non-phi operand fix. (PR #121744)

[David Green via llvm-commits]

================
@@ -292,6 +292,66 @@ InstructionCost VPRecipeBase::computeCost(ElementCount VF,
   llvm_unreachable("subclasses should implement computeCost");
 }
 
+InstructionCost
+VPPartialReductionRecipe::computeCost(ElementCount VF,
+                                      VPCostContext &Ctx) const {
+  std::optional<unsigned> Opcode = std::nullopt;
+  VPRecipeBase *BinOpR = getOperand(0)->getDefiningRecipe();
+  if (auto *WidenR = dyn_cast<VPWidenRecipe>(BinOpR))
+    Opcode = std::make_optional(WidenR->getOpcode());
+
+  VPRecipeBase *ExtAR = BinOpR->getOperand(0)->getDefiningRecipe();
+  VPRecipeBase *ExtBR = BinOpR->getOperand(1)->getDefiningRecipe();
----------------
davemgreen wrote:

Would it be possible for the first operand to have no Def? In that case it could still be accessed in `Ctx.Types.inferScalarType(ExtAR->getOperand(0))`.

https://github.com/llvm/llvm-project/pull/121744