isl_qpolynomial_fold_get_domain_space: extract out *_peek_domain_space
isl_qpolynomial_fold_get_domain_space: extract out *_peek_domain_space

This function will be reused in the next commit.

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

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