isl_aff_project_domain_on_params: extract out drop_domain
isl_aff_project_domain_on_params: extract out drop_domain

This function will be reused in the next commit.

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

inducer/isl-mirror,PollyLabs/isl,PollyLabs/isl,Meinersbur/isl,PollyLabs/isl,PollyLabs/isl,inducer/isl-mirror,Meinersbur/isl,inducer/isl-mirror,Meinersbur/isl,PollyLabs/isl,inducer/isl-mirror,Meinersbur/isl,inducer/isl-mirror