gpu: extract set of accessed array elements from schedule tree
gpu: extract set of accessed array elements from schedule tree

Before, we would extract the set of accessed array elements from the schedule
generated by the AST generator for the host code.
Since we want to perform the kernel analysis before the AST generation,
we need to extract this set from the schedule tree instead.

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

Meinersbur/ppcg,nicolasvasilache/ppcg,nicolasvasilache/ppcg,Meinersbur/ppcg