Propagate size correctly across effects that change output size.

When propagating size information between effects in a phase,
we'd forget to check if the effect wanted to change size
and use that information instead of our own heuristics.
Fix that.

This is currently a no-op, since right now we always break a phase
when an effect changes output size, but there are very real situations
where we'd be fine with not doing so, so this patch paves the way
for that.