gpu_group.c: check_scalar_live_ranges: isolate dependence on gpu_gen
gpu_group.c: check_scalar_live_ranges: isolate dependence on gpu_gen

In a subsequent commit, we will perform the array reference grouping
before the AST generation, which we will need to extract schedule
information from the schedule tree rather than from gpu_gen.
Isolate the dependence on gpu_gen to minimize the changes when
we perform the switch.

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

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