[llvm] [LoopVectorizer] Allow partial reductions to be made in predicated loops (PR #124268)

[Sam Tebbs via llvm-commits]

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@@ -9703,7 +9698,8 @@ void LoopVectorizationPlanner::adjustRecipesForReductions(
     // beginning of the dedicated latch block.
     auto *OrigExitingVPV = PhiR->getBackedgeValue();
     auto *NewExitingVPV = PhiR->getBackedgeValue();
-    if (!PhiR->isInLoop() && CM.foldTailByMasking()) {
+    if (!PhiR->isInLoop() && CM.foldTailByMasking() &&
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SamTebbs33 wrote:

A comment here explaining why partial reductions have a special case would be good.

https://github.com/llvm/llvm-project/pull/124268