isl_output.c: need_exists: take into account dump mode
isl_output.c: need_exists: take into account dump mode

That is, take into account the effect of dump mode inside need_exists
rather than doing that in the caller.

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

nicolasvasilache/isl,PollyLabs/isl,inducer/isl-mirror,tobig/isl,PollyLabs/isl,tobig/isl,tobig/isl,nicolasvasilache/isl,nicolasvasilache/isl,Meinersbur/isl,inducer/isl-mirror,Meinersbur/isl,Meinersbur/isl,inducer/isl-mirror,nicolasvasilache/isl,inducer/isl-mirror,inducer/isl-mirror,nicolasvasilache/isl,Meinersbur/isl,PollyLabs/isl,PollyLabs/isl,PollyLabs/isl,tobig/isl