gpu.c: group_tile: reimplement in terms of mapping to shared/private tile
gpu.c: group_tile: reimplement in terms of mapping to shared/private tile

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

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