isl_ast_codegen.c: foreach_iteration: handle empty domain
isl_ast_codegen.c: foreach_iteration: handle empty domain

Although the set passed to foreach_iteration should never be empty,
it may become empty after intersection with the build domain.
Handle this case specifically since the default procedure of looking
for a lower bound does not work on an empty set.

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

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