This patch does several things:

* Make exprOkForSpeculation ignore evaluatedness of variables
  See the Note [exprOkForSpeculation and evaluated variables]
  This means that the binder-swap transformation no longer
  invaliates the let/app invariant.

* Make exprOkForSpeculation return False for
     DataToTagOp and SeqOp.
  See Note [exprOkForSpeculation and SeqOp/DataToTagOp]

* Remove the 'can_fail' property from dataToTag#; it was
  always a hack (described in the old Note [dataToTag#] in
  primops.txt.pp), and now its not necessary because of the
  fixes above.

* Make SetLevels use exprIsHNF, /not/ exprOkForSpeculation,
  when floating single-alternative cases.  See SetLevels
  Note [Floating single-alternative cases]

* Fix a buglet in FloatIn; probably never bites in practice
  See Note [Dead bindings]

Collectively, these changes finally fix Trac #15696.