proper implementation of isl_union_map_from_range
proper implementation of isl_union_map_from_range

The original implementation would just return the input
because isl_sets and isl_maps have the same internal representation.
This will change in the near future, however, and so we need
to actually compute the range of all the elements in the input.

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

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