Some refactoring of Demand and DmdAnal

This was authored by SPJ and extracted from the "Improve the handling of
used-once stuff" patch by Joachim.
Do not split void functions

This is authored by SPJ, and split out out "Improve the handling of
used-once stuff" by Joachim.
Assign strictness signatures to primitive operations

This patch was authored by SPJ, and extracted from "Improve the handling
of used-once stuff" by Joachim.
Guarding against silly shifts

This patch was authored by SPJ and extracted from "Improve the handling
of used-once stuff" by Joachim.
Improve the handling of used-once stuff

Joachim and I are committing this onto a branch so that we can share it,
but we expect to do a bit more work before merging it onto head.

Nofib staus:
  - Most programs, no change
  - A few improve
  - A couple get worse (cacheprof, tak, rfib)
Investigating the "get worse" set is what's holding up putting this
on head.

The major issue is this.  Consider

    map (f g) ys

where f's demand signature looks like

   f :: <L,C1(C1(U))> -> <L,U> -> .

So 'f' is not saturated.  What demand do we place on g?
Answer
        C(C1(U))
That is, the inner C1 should stay, even though f is not saturated.

I found that this made a significant difference in the demand signatures
inferred in GHC.IO, which uses lots of higher-order exception handlers.

I also had to add used-once demand signatures for some of the
'catch' primops, so that we know their handlers are only called once.