isl_set_get_simple_fixed_box_hull: extract out fixed_box_as_map
isl_set_get_simple_fixed_box_hull: extract out fixed_box_as_map

This will be reused in the next commit.

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl