Be less conservative about constant propagation from layer to layer.
It used to do so if the output of the earlier layer was never used in the
upstream layer; I suspected that it's only important for it to never be
written, of course who care if it's read by the upstream layer.  Looks like
this change doesn't break anything, while increasing the number of
connections it can remove by constant propagation.

But, also, DO make sure it's not a parameter that can be overridden by
geometry. I think that might have been a latent bug.