[LV] Simplify !UserVF.isZero() -> UserVF (NFC).

Address post-commit comment for b8dccb7 to simplify code.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7014,7 +7014,7 @@ LoopVectorizationPlanner::plan(ElementCount UserVF, unsigned UserIC) {
 
   ElementCount MaxUserVF =
       UserVF.isScalable() ? MaxFactors.ScalableVF : MaxFactors.FixedVF;
-  if (!UserVF.isZero()) {
+  if (UserVF) {
     if (!ElementCount::isKnownLE(UserVF, MaxUserVF)) {
       reportVectorizationInfo(
           "UserVF ignored because it may be larger than the maximal safe VF",