Note [RuntimeRep polymorphism]

Generally speaking, you can't be polymorphic in `RuntimeRep`.  E.g
   f :: forall (rr::RuntimeRep) (a::TYPE rr). a -> [a]
   f = /\(rr::RuntimeRep) (a::rr) \(a::rr). ...
This is no good: we could not generate code for 'f', because the
calling convention for 'f' varies depending on whether the argument is
a a Int, Int#, or Float#.  (You could imagine generating specialised
code, one for each instantiation of 'rr', but we don't do that.)

Certain functions CAN be runtime-rep-polymorphic, because the code
generator never has to manipulate a value of type 'a :: TYPE rr'.

* error :: forall (rr::RuntimeRep) (a::TYPE rr). String -> a
  Code generator never has to manipulate the return value.

* unsafeCoerce#, defined in Desugar.mkUnsafeCoercePair:
  Always inlined to be a no-op
     unsafeCoerce# :: forall (r1 :: RuntimeRep) (r2 :: RuntimeRep)
                             (a :: TYPE r1) (b :: TYPE r2).
                             a -> b

* Unboxed tuples, and unboxed sums, defined in GHC.Builtin.Types
  Always inlined, and hence specialised to the call site
     (#,#) :: forall (r1 :: RuntimeRep) (r2 :: RuntimeRep)
                     (a :: TYPE r1) (b :: TYPE r2).
                     a -> b -> TYPE ('TupleRep '[r1, r2])