isl_aff.c: replace_by_nan: extract out nan_on_domain_set
isl_aff.c: replace_by_nan: extract out nan_on_domain_set

This function will be reused in the next commit.

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

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