isl_union_*_transform_data: extract out isl_union_*_transform_control
isl_union_*_transform_data: extract out isl_union_*_transform_control

This will make it possible to pass an isl_union_*_transform_control
object to isl_union_*_transform_space, making it easier
to extend its functionality.

Signed-off-by: Sven Verdoolaege <dd860110a62b19214c4ee03aec0abffecb4e86b8@cerebras.net>

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