isl_map_from_pw_multi_aff: preserve rational flag

isl_map_from_pw_multi_aff: preserve rational flag

In particular, if the domain of the input is rational, then the result
should be rational as well.

This fixes the computation of rational lexicographic optima,
which was (sort of) broken by 4757a6d (isl_map_partial_lexopt_aligned:
compute result as isl_pw_multi_aff first, Tue Mar 15 16:16:49 2016 +0100).
In particular, for the input

	{ rat: [i] : 1 <= 2i <= 9 or i >= 10 }

the partial results would get converted to integer maps,
but since the minimum is a rational value (1/2), the result
would be considered to be empty.  By preserving the rational
flag in isl_map_from_pw_multi_aff, the rational solution is preserved.

Note that the result was not exactly correct before the above-mentioned
commit either.  In particular, before that commit, the result produced
was

	{ rat: [i] : 2i = 1; rat: [10] }

rather than the correct

	{ rat: [i] : 2i = 1 }.

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