[Bug libstdc++/64903] is_partitioned should not apply a predicate more than (last - first) times

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64903

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #1)
> I think the problem is that we end up testing the partition point twice.
> Here's a completely untested patch that avoids that:

Has this patch been tested and applied?