[llvm] [LV] Use VPReductionRecipe for partial reductions (PR #144908)

[Sam Tebbs via llvm-commits]

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@@ -7050,7 +7050,8 @@ static bool planContainsAdditionalSimplifications(VPlan &Plan,
       }
       // The VPlan-based cost model is more accurate for partial reduction and
       // comparing against the legacy cost isn't desirable.
-      if (isa<VPPartialReductionRecipe>(&R))
+      if (auto *VPR = dyn_cast<VPReductionRecipe>(&R);
+          VPR && VPR->isPartialReduction())
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SamTebbs33 wrote:

Are you talking about [here](https://github.com/llvm/llvm-project/pull/144908/files#diff-da321d454a7246f8ae276bf1db2782bf26b5210b8133cb59e4d7fd45d0905decR8354)? That's different because it's checking if it's either a `VPReductionRecipe` or `VPReductionPHIRecipe` and you can't declare two variables in an if-statement. I think this method is cleaner when there's just one variable to declare since you don't have to check the type and then cast, you just cast.

https://github.com/llvm/llvm-project/pull/144908