isl_basic_map_drop_constraints{_not,}_involving: add back div constraints

isl_basic_map_drop_constraints{_not,}_involving: add back div constraints

The div constraints are assumed to always be expressed by the constraints
of a basic map.  They may however get removed if they turn out to be
redundant with respect to some other constraints.  When we are removing
constraints in these two functions, we may be removing constraints
that helped to make the div constraints redundant, leaving the
div constraints unexpressed.  Add back all div constraints to make
sure that they are all expressed by the basic map.

Reported-by: Roman Gareev <gareevroman@gmail.com>
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>