Floating lazy primops

[David Feuer]

I opened up https://phabricator.haskell.org/D2987 to mark 
reallyUnsafePtrEquality# can_fail, but in the process I realized a couple 
things.

1. Part of https://phabricator.haskell.org/rGHC5a9a1738023a may actually not 
have been such a hot idea after all (although it certainly sounded good at the 
time). Are there actually any primops with lifted arguments where we *want* 
speculation? Perhaps the most important primop to consider is seq#, which is 
(mysteriously?) marked neither can_fail nor has_side_effects, but another to 
look at is unpackClosure#, which seems likely to give different results before 
and after forcing. Most other primops with lifted arguments are marked 
has_side_effects, and therefore won't be floated out anyway.

2. If dataToTag# is marked can_fail (an aspect of https://
phabricator.haskell.org/rGHC5a9a1738023a), is it still possible for it to end 
up being applied to an unevaluated argument? If not, perhaps the CorePrep 
specials can be removed altogether.

David Feuer