[LV] update comment following 63d8058 (NFC) (#91120)

Address a review comment post landing 63d8058 (LoopVectorize: guard
appending InstsToScalarize; fix bug) to update a comment.

Author: artagnon

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -5813,10 +5813,11 @@ void LoopVectorizationCostModel::collectInstsToScalarize(ElementCount VF) {
     for (Instruction &I : *BB)
       if (isScalarWithPredication(&I, VF)) {
         ScalarCostsTy ScalarCosts;
-        // Do not apply discount if scalable, because that would lead to
-        // invalid scalarization costs.
-        // Do not apply discount logic if hacked cost is needed
-        // for emulated masked memrefs.
+        // Do not apply discount logic for:
+        // 1. Scalars after vectorization, as there will only be a single copy
+        // of the instruction.
+        // 2. Scalable VF, as that would lead to invalid scalarization costs.
+        // 3. Emulated masked memrefs, if a hacked cost is needed.
         if (!isScalarAfterVectorization(&I, VF) && !VF.isScalable() &&
             !useEmulatedMaskMemRefHack(&I, VF) &&
             computePredInstDiscount(&I, ScalarCosts, VF) >= 0)