Note [Empty case alternatives]

The alternatives of a case expression should be exhaustive.  But
this exhaustive list can be empty!

* A case expression can have empty alternatives if (and only if) the
  scrutinee is bound to raise an exception or diverge. When do we know
  this?  See Note [Bottoming expressions] in GHC.Core.Utils.

* The possibility of empty alternatives is one reason we need a type on
  the case expression: if the alternatives are empty we can't get the
  type from the alternatives!

* In the case of empty types (see Note [Bottoming expressions]), say
    data T
  we do NOT want to replace
    case (x::T) of Bool {}   -->   error Bool "Inaccessible case"
  because x might raise an exception, and *that*'s what we want to see!
  (#6067 is an example.) To preserve semantics we'd have to say
     x `seq` error Bool "Inaccessible case"
  but the 'seq' is just such a case, so we are back to square 1.

* We can use the empty-alternative construct to coerce error values from
  one type to another.  For example

    f :: Int -> Int
    f n = error "urk"

    g :: Int -> (# Char, Bool #)
    g x = case f x of { 0 -> ..., n -> ... }

  Then if we inline f in g's RHS we get
    case (error Int "urk") of (# Char, Bool #) { ... }
  and we can discard the alternatives since the scrutinee is bottom to give
    case (error Int "urk") of (# Char, Bool #) {}

  This is nicer than using an unsafe coerce between Int ~ (# Char,Bool #),
  if for no other reason that we don't need to instantiate the (~) at an
  unboxed type.

* We treat a case expression with empty alternatives as trivial iff
  its scrutinee is (see GHC.Core.Utils.exprIsTrivial).  This is actually
  important; see Note [Empty case is trivial] in GHC.Core.Utils

* We lower empty cases in GHC.CoreToStg.coreToStgExpr to an eval on the
  scrutinee.

Historical Note: We used to lower EmptyCase in CorePrep by way of an
unsafeCoercion on the scrutinee, but that yielded panics in CodeGen when
we were beginning to eta expand in arguments, plus required to mess with
heterogenously-kinded coercions. It's simpler to stick to it just a bit longer.