isl_*_eliminate: perform integer elimination

isl_*_eliminate: perform integer elimination

The original code would perform the elimination "rationally" by
simply applying Fourier-Motzkin on them.  If the input
is an integer set or map, this could drop information on other dimensions.
For example, eliminating the second dimension of

        { [i, j] : i = 2 j }

would result in

        { [i, j] }

while it should result in

        { [i, j] : exists a : i = 2 a }

We now perform an integer elimination (on integer sets and maps).

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