[llvm] [VPlan] Remove legacy costing inside VPBlendRecipe::computeCost (PR #171846)

[Ramkumar Ramachandra via llvm-commits]

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@@ -7081,6 +7081,11 @@ static bool planContainsAdditionalSimplifications(VPlan &Plan,
           return true;
       }
 
+      // The legacy cost model costs non-header phis with a scalar VF as a phi,
+      // but scalar unrolled VPlans will have VPBlendRecipes which emit selects.
+      if (VF.isScalar() && isa<VPBlendRecipe>(&R))
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artagnon wrote:

Hm, should we be using the stronger vputils::onlyFirstLaneUsed instead of VF.isScalar? Not sure if the current patch will result in mismatches?

https://github.com/llvm/llvm-project/pull/171846