Representation-polymorphism checks for primops check too many arguments

The following program is currently rejected:

blart :: forall (r :: RuntimeRep) (a :: TYPE r). a -> a
blart = raise# 5

error:
    Cannot use function with representation-polymorphic arguments:
      raise# :: Integer -> a -> a
    (Note that representation-polymorphic primops,
    such as 'coerce' and unboxed tuples, are eta-expanded
    internally because they must occur fully saturated.
    Use -fprint-typechecker-elaboration to display the full expression.)
    Representation-polymorphic arguments: a :: TYPE r
   |
13 | blart = raise# 5
   |         ^^^^^^

The problem is that the code that checks for disallowed representation-polymorphism when instantiating primops, which is supposed to catch cases like:

foo :: forall (r :: RuntimeRep) (a :: TYPE r) b. a -> b
foo = unsafeCoerce#

ends up checking all the function arguments in the return type, even past the primop's arity.

I will be fixing this as part of !6164 (closed).