Do not forget CPR information after an IO action

but do forget about certain divergence, if required. Fixes one part of
ticket #8598.

The added function (deferAfterIO) can maybe be merged with existing
code, but given the ongoing work in the nested-cpr branch, I defer that
work.
Rename topDmdType to nopDmdType

because topDmdType is ''not'' the top of the lattice, as it puts an
implicit absent demand on free variables, but Abs is the bottom of the
Usage lattice.

Why nopDmdType? Becuase it is the demand of doing nothing: Everything
lazy, everything absent, no definite divergence.