Refactor the simplifier's treatment of case expressions

Refactor the simplifier's treatment of case expressions

(NB: this patch could conceivably require some bits of the
following SpecConstr patch to compile cleanly.  It's conceptually
independent, but I'm not 100% certain that I've included all
the necessary bits here.)

This patch cleans up the simplifier's handling of various
otimisations for case expressions, notably
  - case elimination (discarding the case altogether)
  - merging identical alternatives
  - discarding impossible alternative
  - merging nested cases

Previously this was partly handled before, and partly after,
simplifying the case alternatives. The trouble with that is
that the dead-ness information on the case binders gets munged
during simplification, and that turned out to mean that
case elmination essentially never happened -- stupid.

Now I've moved it all to before simplifying the alterntives.
In fact this reduces the amount of code, I think, and it's
certainly tidier.  I don't think there is any loss.