This patch implements two related changes, both inspired by
the discussion on Trac #13027, comment:23:

* exprOkForSpeculation (op# a1 .. an), where op# is a primop,
  now skips over arguments ai of lifted type.  See the comments
  at Note [Primops with lifted arguments] in CoreUtils.

  There is no need to treat dataToTag# specially any more.

* dataToTag# is now treated as a can-fail primop.  See
  Note [dataToTag#] in primops.txt.pp

I don't expect this to have a visible effect on anything, but
it's much more solid than before.