isl_tab_from_basic_map: preserve all constraints in input when tracking

isl_tab_from_basic_map: preserve all constraints in input when tracking

If the user wants to keep track of a basic map representation of the tableau,
then we need to make sure that the initial tableau corresponds exactly to
the input.  In particular, we need to make sure that no (redundant) constraints
are removed.

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