gpu_group.c: check_scalar_live_ranges: avoid composing schedule with itself
gpu_group.c: check_scalar_live_ranges: avoid composing schedule with itself

Composing a schedule with its own inverse results in a data structure
of quadratic size.  Use (a different representation of) the schedule
itself instead.

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

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