isl_sched_node: use isl_pw_multi_aff to represent decompression
isl_sched_node: use isl_pw_multi_aff to represent decompression

In an upcoming commit, an additional domain compression
may get performed that may result in a piecewise decompression.
Prepare for this possibility.

It is not immediately obvious which of isl_pw_multi_aff or
isl_multi_pw_aff would be the most appropriate representation,
but isl_pw_multi_aff requires the least amount of changes
since a isl_map_preimage_range_pw_multi_aff is already available,
while isl_map_preimage_range_multi_pw_aff is not.

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

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