QAIV: speed up isRowHidden for the case where no rows are hidden.

isPersistent(index) is a linear search, which we can avoid altogether
(as well as the creation of a persistent model index, is idx is indeed persistent)
when the row-hiding feature isn't used at all.

Change-Id: I07488dddf56e299806cc29749e633da00115fc30
Reviewed-by: Jan Arve Sæther <jan-arve.saether@theqtcompany.com>

src/widgets/itemviews/qtreeview_p.h

@@ -210,6 +210,8 @@ public:
     QSet<QPersistentModelIndex> hiddenIndexes;
 
     inline bool isRowHidden(const QModelIndex &idx) const {
+        if (hiddenIndexes.isEmpty())
+            return false;
         //We first check if the idx is a QPersistentModelIndex, because creating QPersistentModelIndex is slow
         return isPersistent(idx) && hiddenIndexes.contains(idx);
     }