gpu: extract effective grid size from schedule tree

gpu: extract effective grid size from schedule tree

Before, we would extract the effective grid size based on the domain
of the schedule generated by the AST generator for the host code.
Since we want to insert a context node in the schedule tree expressing
the bounds on the block identifiers,
we need to extract this set from the schedule tree instead.

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