Assertion failure with levity polymorphism

The following code, taken from T13233, triggers an assertion failure in HEAD.

{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE UnboxedTuples #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE MagicHash #-}
module Bug where

import GHC.Exts (TYPE, RuntimeRep, Weak#, State#, RealWorld, mkWeak# )

-- It used to be that primops has no binding. However, as described in
-- Note [Primop wrappers] in PrimOp we now rewrite unsaturated primop
-- applications to their wrapper, which allows safe use of levity polymorphism.
primop :: forall (rep :: RuntimeRep) (a :: TYPE rep) b c.
          a -> b -> (State# RealWorld -> (# State# RealWorld, c #))
       -> State# RealWorld -> (# State# RealWorld, Weak# b #)
primop = mkWeak#

WARNING: file compiler/coreSyn/CoreArity.hs, line 1122
  4 forall (q :: RuntimeRep) (a :: TYPE q) b c.
    a
    -> b
    -> (State# RealWorld -> (# State# RealWorld, c #))
    -> State# RealWorld
    -> (# State# RealWorld, Weak# b #)
  mkWeak#
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1248:29 in ghc:Outputable
        warnPprTrace, called at compiler/coreSyn/CoreArity.hs:1122:11 in ghc:CoreArity
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.11.0.20200210:
	ASSERT failed!
  primop
  Id arity: 4
  STG arity: 0
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1187:37 in ghc:Outputable
        pprPanic, called at compiler/utils/Outputable.hs:1257:5 in ghc:Outputable
        assertPprPanic, called at compiler/GHC/CoreToStg.hs:314:66 in ghc:GHC.CoreToStg

This is not caught by the test itself, because it fails to compile due to other code present in this file. As a part of this ticket, let's split it into two (and remove mkWeak# from T13233_elab).