isl_basic_map_foreach_lexopt: properly merge partial solutions

isl_basic_map_foreach_lexopt: properly merge partial solutions

If we detect two identical solutions in adjacent parts of the search space,
then we merge them into a single solution.  The combined domain may however
have fewer existentially quantified variables and so we also need to adjust
the matrix representing the solution on that domain to no longer refer
to the extra existentially quantified variables.  Otherwise, the code
in sol_for_add could end up accessing elements of aff->v outside its bounds.
Note that it is safe to remove the columns that refer to the extra
existentially quantified variables since their coefficients need to be zero
for the two partial solutions to be considered identical.

Reported-by: Andreas Kloeckner <lists@informa.tiker.net>
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>