isl_basic_map_drop_constraints_involving: move out adding div constraints

isl_basic_map_drop_constraints_involving: move out adding div constraints

The call to isl_basic_map_add_known_div_constraints was added in
isl-0.13-32-gbec90cd4ee (isl_basic_map_drop_constraints{_not,}_involving:
add back div constraints, Mon Aug 4 11:46:24 2014 +0200), where
one call was added to isl_basic_map_drop_constraints_not_involving_dims,
while the other call was added to isl_basic_map_drop_constraints_involving
instead of isl_basic_map_drop_constraints_involving_dims.
isl_basic_map_drop_constraints_involving is currently only called
directly on objects without integer division, so there is no need
to add back any div constraints for callers of
isl_basic_map_drop_constraints_involving that do not go through
isl_basic_map_drop_constraints_involving_dims.
Furthermore, isl_basic_map_drop_constraints_involving will
be reused in an upcoming commit to drop the div constraints
of a redundant integer division, in which case adding
div constraints for other integer divisions is pure overhead.

Signed-off-by: Sven Verdoolaege <sven.verdoolaege@gmail.com>