isl_pw_multi_aff_from_map: detect easy floors directly from constraints

isl_pw_multi_aff_from_map: detect easy floors directly from constraints

If the input map has constraints of the form

	{ [i] -> [j] : i - 1 <= 2 j <= i }

then we can easily detect that j can be taken to be equal to floor(i / 2).
Detecting these conditions directly from the constraints saves us
from having to compute a minimum on this dimension
using parametric integer programming.  Not only can such an operation
be fairly costly, it can also lead to needless subdivisions if there
are any other constraints on the dimension to be minimized.

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