Note [Floating single-alternative cases]

Consider this:
  data T a = MkT !a
  f :: T Int -> blah
  f x vs = case x of { MkT y ->
             let f vs = ...(case y of I# w -> e)...f..
             in f vs

Here we can float the (case y ...) out, because y is sure
to be evaluated, to give
  f x vs = case x of { MkT y ->
           case y of I# w ->
             let f vs = ...(e)...f..
             in f vs

That saves unboxing it every time round the loop.  It's important in
some DPH stuff where we really want to avoid that repeated unboxing in
the inner loop.

Things to note:

 * The test we perform is exprIsHNF, and /not/ exprOkForSpeculation.

     - exprIsHNF catches the key case of an evaluated variable

     - exprOkForSpeculation is /false/ of an evaluated variable;
       See Note [exprOkForSpeculation and evaluated variables] in GHC.Core.Utils
       So we'd actually miss the key case!

     - Nothing is gained from the extra generality of exprOkForSpeculation
       since we only consider floating a case whose single alternative
       is a DataAlt   K a b -> rhs

 * We can't float a case to top level

 * It's worth doing this float even if we don't float
   the case outside a value lambda.  Example
     case x of {
       MkT y -> (case y of I# w2 -> ..., case y of I# w2 -> ...)
   If we floated the cases out we could eliminate one of them.

 * We only do this with a single-alternative case