isl_aff_ceil: compute ceil(e/m) as floor((e + m - 1)/m)

isl_aff_ceil: compute ceil(e/m) as floor((e + m - 1)/m)

The original implementation would compute ceil(e/m) as -floor(-e/m).
Although both are correct, floor((e + m - 1)/m) is probably less
surprising.

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