list_select_outer_band: update tile_first after extending prefix
list_select_outer_band: update tile_first after extending prefix

tile_first refers to the position of the selected tilable band
and prefix contains the schedule dimensions up to that band.
They therefore need to be updated together.

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

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