isl_basic_set_reduced_basis: fix value in directions with only one integer value

isl_basic_set_reduced_basis: fix value in directions with only one integer value

If two or more directions have a width that is smaller than one,
i.e., such that the only one integer value can be attained,
then the standard algorithm would still try to order these
directions.  However, if the first directions only admit one
value, then it doesn't matter in which order they are put.
We therefore keep track of such directions explicitly and
also set the value in the tableau to the fixed value, rather
than keeping the rational interval with only one integer value.