DenotTransformers that change the signature of their denotation's info must go
after erasure. The reason is that denotations are permanently referred to by
TermRefs which contain a signature. If the signature of a symbol would change,
all refs to it would become outdated - they could not be dereferenced in the
new phase.
After erasure, signature changing denot-transformers are OK because signatures
are never recomputed later than erasure.
