isl_pw_*_get_space: extract out isl_pw_*_peek_space
isl_pw_*_get_space: extract out isl_pw_*_peek_space

This function will be reused in the next commit.

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

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