isl_pw_multi_aff_from_map: detect strides in output dimensions

isl_pw_multi_aff_from_map: detect strides in output dimensions

If the input map is of the form

	{ [i] -> [j] : exists a: j = 4 a and i - 1 <= j <= i }

then the detection mechanism of the previous commit will not
be able to read off the definition of j directly from the constraints.
However, if we plug in j = 4 j', we obtain

	{ [i] -> [j'] : i - 1 <= 4 j' <= i }

from which we can read off j' = floor(i/4), which can then be plugged
back into j = 4 j' to obitain j = 4 * floor(i/4).

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