isl_basic_map_plain_cmp: specifically handle unknown local variables

isl_basic_map_plain_cmp: specifically handle unknown local variables

Local variables without a known integer division expression
are sometimes created with zero values for all coefficients,
but this is not guaranteed.
This could lead to two basic map being considered not obviously equal
to each other even though they have exactly the same constraints.
In any case, there is no point in comparing the coefficients
of local variables without a known integer division expression.

Note that the new test case also passes without this commit
because the local variables just happen to get created
using all zero coefficients.

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