Eliminate case-of-cast

Eliminate case-of-cast

Note [Case of cast]
~~~~~~~~~~~~~~~~~~~
Consider 	case (v `cast` co) of x { I# ->
		... (case (v `cast` co) of {...}) ...
We'd like to eliminate the inner case.  We can get this neatly by
arranging that inside the outer case we add the unfolding
	v |-> x `cast` (sym co)
to v.  Then we should inline v at the inner case, cancel the casts,
and away we go

This patch does the job, fixing a performance hole reported by Roman.