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

[Florian Hahn via llvm-commits]

================
@@ -2440,6 +2446,50 @@ class VPReductionPHIRecipe : public VPHeaderPHIRecipe,
   bool isInLoop() const { return IsInLoop; }
 };
 
+/// A recipe for forming partial reductions. In the loop, an accumulator and
+/// vector operand are added together and passed to the next iteration as the
+/// next accumulator. After the loop body, the accumulator is reduced to a
+/// scalar value.
+class VPPartialReductionRecipe : public VPSingleDefRecipe {
+  unsigned Opcode;
+
+public:
+  VPPartialReductionRecipe(Instruction *ReductionInst, VPValue *Op0,
+                           VPValue *Op1)
+      : VPPartialReductionRecipe(ReductionInst->getOpcode(), Op0, Op1,
+                                 ReductionInst) {}
+  VPPartialReductionRecipe(unsigned Opcode, VPValue *Op0, VPValue *Op1,
+                           Instruction *ReductionInst = nullptr)
+      : VPSingleDefRecipe(VPDef::VPPartialReductionSC,
+                          ArrayRef<VPValue *>({Op0, Op1}), ReductionInst),
+        Opcode(Opcode) {
+    assert(isa<VPReductionPHIRecipe>(getOperand(1)->getDefiningRecipe()) &&
+           "Unexpected operand order for partial reduction recipe");
+  }
+  ~VPPartialReductionRecipe() override = default;
+  VPPartialReductionRecipe *clone() override {
----------------
fhahn wrote:

```suggestion
  ~VPPartialReductionRecipe() override = default;
  
  VPPartialReductionRecipe *clone() override {
```

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