Add comments to explain why we skip scalar cost.

Author: ElvisWang123

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -4327,6 +4327,9 @@ void LoopVectorizationPlanner::emitInvalidCostRemarks(
   SmallVector<RecipeVFPair> InvalidCosts;
   for (const auto &Plan : VPlans) {
     for (ElementCount VF : Plan->vectorFactors()) {
+      // The VPlan-based cost model is designed for computing vector cost.
+      // Quering VPlan-based cost model with scarlar VF will cause some error
+      // because we expect the VF is vector for most of the widen recipes.
       if (VF.isScalar())
         continue;