isl_aff.c: upa_pb_upma: use isl_union_pw_multi_aff_foreach_pw_multi_aff
isl_aff.c: upa_pb_upma: use isl_union_pw_multi_aff_foreach_pw_multi_aff

Do so instead of depending on the internal representation
of an isl_union_pw_multi_aff.

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

nicolasvasilache/isl,inducer/isl-mirror,inducer/isl-mirror,inducer/isl-mirror,Distrotech/isl,KangDroidSMProject/ISL,PollyLabs/isl,KangDroidSMProject/ISL,Meinersbur/isl,Meinersbur/isl,UBERTC/isl,tobig/isl,KangDroidSMProject/ISL,Meinersbur/isl,Distrotech/isl,inducer/isl-mirror,nicolasvasilache/isl,inducer/isl-mirror,Meinersbur/isl,tobig/isl,KangDroidSMProject/ISL,UBERTC/isl,nicolasvasilache/isl,PollyLabs/isl,BenzoSM/isl,BenzoSM/isl,tobig/isl,PollyLabs/isl,Distrotech/isl,PollyLabs/isl,Distrotech/isl,nicolasvasilache/isl,nicolasvasilache/isl,Distrotech/isl,UBERTC/isl,tobig/isl,PollyLabs/isl,UBERTC/isl,BenzoSM/isl,BenzoSM/isl,BenzoSM/isl,KangDroidSMProject/ISL