Note [Check the output scrutinee for exprIsHNF]

Consider this:
  case x of y {
    A -> ....(case y of alts)....
  }

Because of the binder-swap, the inner case will get substituted to
(case x of ..).  So when testing whether the scrutinee is in HNF we
must be careful to test the *result* scrutinee ('x' in this case), not
the *input* one 'y'.  The latter *is* in HNF here (because y is
evaluated), but the former is not -- and indeed we can't float the
inner case out, at least not unless x is also evaluated at its binding
site.  See #5453.

That's why we apply exprIsHNF to scrut' and not to scrut.

See Note [Floating single-alternative cases] for why
we use exprIsHNF in the first place.