IO hack in demand analyzer can miss I/O

If someone digs into the IO type, they could (with good reason, even!) decide that they want to produce something other than an unboxed pair. Here's an example, imitating StateT st IO a in a way that's sure to be unboxed well:

newtype IOState st a = IOState
  { unIOState :: State# RealWorld
              -> st
              -> (# State# RealWorld, st, a #) }

This won't trip io_hack_reqd, so demand analysis won't see it as I/O, and everything will be wrong. Urk. Furthermore, in the future, we may allow newtypes around unlifted types, in which case this stuff gets even messier.

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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