isl_set_dim_residue_class: correctly consider all basic sets in the set

isl_set_dim_residue_class: correctly consider all basic sets in the set

This commit fixes two closely related problems.
Due to a typo, isl_set_dim_residue_class would only consider one of
the basic sets in the input set.  The information obtained from the
individual basic sets was also combined incorrectly.
In particular, the code would insist that all basic sets have the
same residue if they have the same modulo and not consider adjusting
the modulo of the combined basic sets to accommodate for the difference
in residues.

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