QTreeView: 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: I55ef204dd9b79a89d51d5d3aacc1c110d37a4ea0
(cherry picked from qtbase/cd824ee79188b889df85d4d648a625589655e15e)
Reviewed-by: Jan Arve Sæther <jan-arve.saether@theqtcompany.com>