isl_basic_set_expand_divs: only add div constraints for known divs

isl_basic_set_expand_divs: only add div constraints for known divs

When called from isl_basic_set_opt_lp_val_aligned or isl_basic_set_opt,
the existentially quantified variables that are introduced, always
have an explicit representation, but this may not necessarily be
the case when called from contains_with_expanded_divs.
Adding div constraints based on an unknown div constructs
constraints from stale data, which may therefore not make any sense.
In practice, this does not cause any major damage because
the two constraints that end up being added conflict with each other.
They therefore cannot both be valid for the other disjunct and
therefore result at most in missed coalescing opportunities.

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>