isl_aff_nan_on_domain: extract out isl_aff_set_nan
isl_aff_nan_on_domain: extract out isl_aff_set_nan

The extracted function will be reused from within isl_aff_div
in an upcoming commit.

Signed-off-by: Sven Verdoolaege <dd860110a62b19214c4ee03aec0abffecb4e86b8@cerebras.net>
Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

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