[llvm] [LV] Handle partial reductions with predication (PR #194859)

[Benjamin Maxwell via llvm-commits]

================
@@ -6543,13 +6546,28 @@ static void transformToPartialReduction(const VPPartialReductionChain &Chain,
   }
 
   // Check if WidenRecipe is the final result of the reduction. If so look
-  // through selects for predicated reductions.
-  VPValue *Cond = nullptr;
-  VPValue *ExitValue = cast_or_null<VPInstruction>(
-      findUserOf(WidenRecipe, m_Select(m_VPValue(Cond), m_Specific(WidenRecipe),
-                                       m_Specific(RdxPhi))));
-  bool IsLastInChain = RdxPhi->getBackedgeValue() == WidenRecipe ||
-                       RdxPhi->getBackedgeValue() == ExitValue;
+  // through the select recipes introduced by tail-folding or predicated
+  // reductions for the BinOp's exit value.
+  VPValue *TailFoldCond = nullptr;
+  auto *ExitValue = cast_or_null<VPInstruction>(findUserOf(
+      WidenRecipe, m_Select(m_VPValue(TailFoldCond), m_Specific(WidenRecipe),
+                            m_Specific(RdxPhi))));
+
+  VPValue *BlendCond = Chain.Blend ? Chain.Blend->getMask(0) : nullptr;
+  VPValue *Cond = TailFoldCond;
+
+  if (TailFoldCond && BlendCond) {
+    VPBuilder Builder(WidenRecipe);
+    Cond = Builder.createLogicalAnd(TailFoldCond, BlendCond,
+                                    WidenRecipe->getDebugLoc());
+  } else if (BlendCond) {
+    Cond = BlendCond;
+  }
+
+  bool IsLastInChain =
+      RdxPhi->getBackedgeValue() == WidenRecipe ||
+      RdxPhi->getBackedgeValue() == ExitValue ||
+      (Chain.Blend && RdxPhi->getBackedgeValue() == Chain.Blend);
----------------
MacDue wrote:

nit: (this is still valid for a null Chain.Blend)
```suggestion
      RdxPhi->getBackedgeValue() == Chain.Blend;
```

https://github.com/llvm/llvm-project/pull/194859