Note [exprOkForSpeculation and type classes]

Consider (#22745, #15205)

  \(d :: C a b). case eq_sel (sc_sel d) of
                   (co :: t1 ~# t2) [Dead] ->  blah

We know that
* eq_sel's argument (sc_sel d) has dictionary type, so it definitely terminates
  (again Note [NON-BOTTOM-DICTS invariant] in GHC.Core)
* eq_sel is simply a superclass selector, and hence is fast
* The field that eq_sel picks is of unlifted type, and hence can't be bottom
  (remember the dictionary argument itself is non-bottom)

So we can treat (eq_sel (sc_sel d)) as ok-for-speculation.  We must check

a) That the function is a class-op, with IdDetails of ClassOpId

b) That the result type of the class-op is terminating or unlifted.  E.g. for
     class C a => D a where ...
     class C a where { op :: a -> a }
   Since C is represented by a newtype, (sc_sel (d :: D a)) might
   not be terminating.

Rather than repeatedly test if the result of the class-op is a
terminating/unlifted type, we cache it as a field of ClassOpId. See
GHC.Types.Id.Make.mkDictSelId for where this field is initialised.