isl_scheduler.c: carry_dependences: extract out compute_carrying_sol
isl_scheduler.c: carry_dependences: extract out compute_carrying_sol

This isolates the part of carry_dependences that deals with
the validity constraints.  This will make it easier to change
the way those validity constraints are collected in the next commit.

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

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