isl_multi_*_drop_dims: extract out isl_multi_*_drop_output_dims
isl_multi_*_drop_dims: extract out isl_multi_*_drop_output_dims

The handling of output dimensions is quite different from
that of other dimensions, so it is better to separate it out.

Signed-off-by: Sven Verdoolaege <dd860110a62b19214c4ee03aec0abffecb4e86b8@cerebras.net>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl