isl_scheduler.c: compute_size: extract out isolate
isl_scheduler.c: compute_size: extract out isolate

This function will be reused to compute an expression
for a dimension with a fixed value in terms of the other dimensions.

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

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