isl_scheduler.c: compute_sizes_and_max: extract out compute_sizes
isl_scheduler.c: compute_sizes_and_max: extract out compute_sizes

This will make it easier to take remedial action in case
one of the sizes turns out to be zero in the next commit.

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

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