[LV] Clarify comment for selectVectorizationFactor (NFC).

The comment is stale, as UserVF is handled before selectVectorizationFactor
is called. Clarify the comment by remove the mention of UserVF.

Suggested as independent improvement in D143938.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -1193,9 +1193,7 @@ class LoopVectorizationCostModel {
   bool runtimeChecksRequired();
 
   /// \return The most profitable vectorization factor and the cost of that VF.
-  /// This method checks every VF in \p CandidateVFs. If UserVF is not ZERO
-  /// then this vectorization factor will be selected if vectorization is
-  /// possible.
+  /// This method checks every VF in \p CandidateVFs.
   VectorizationFactor
   selectVectorizationFactor(const ElementCountSet &CandidateVFs);