[llvm] [VPlan] Introduce ComputeReductionResult VPInstruction opcode. (PR #70253)

[Alexey Bataev via llvm-commits]

================
@@ -403,6 +404,138 @@ Value *VPInstruction::generateInstruction(VPTransformState &State,
     Builder.GetInsertBlock()->getTerminator()->eraseFromParent();
     return CondBr;
   }
+  case VPInstruction::ComputeReductionResult: {
+    if (Part != 0)
+      return State.get(
+          this, VPIteration(State.UF - 1, VPLane::getLastLaneForVF(State.VF)));
+
+    auto *PhiR = dyn_cast<VPReductionPHIRecipe>(getOperand(0));
+    PHINode *OrigPhi = cast<PHINode>(PhiR->getUnderlyingValue());
----------------
alexey-bataev wrote:

```suggestion
    auto *OrigPhi = cast<PHINode>(PhiR->getUnderlyingValue());
```

https://github.com/llvm/llvm-project/pull/70253