isl_input.c: nan_on_domain: use isl_pw_aff_nan_on_domain_space
isl_input.c: nan_on_domain: use isl_pw_aff_nan_on_domain_space

This simplifies the code.

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

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