isl_set_unbind_params_insert_domain: extract out isl_set_check_is_set
isl_set_unbind_params_insert_domain: extract out isl_set_check_is_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