GHC panic when fiddling with quantified constraints

Summary

Given the following code:

{-# LANGUAGE
    QuantifiedConstraints
  , StandaloneKindSignatures
  , TypeOperators
  , GADTs
  , ConstraintKinds
  , RankNTypes
  , UndecidableInstances
  , ImpredicativeTypes
#-}
module Typelevel.Constraint.Repro where

import Data.Kind (Constraint, Type)

type Dict :: Constraint -> Type
data Dict c
  where
  Dict :: c => Dict c

type (⇒) :: Constraint -> Constraint -> Constraint
type c ⇒ d = c => d
infixr 0 ⇒

type (\/) :: Constraint -> Constraint -> Constraint
type a \/ b = (forall r. (a ⇒ r, b ⇒ r) ⇒ r)
infixr 5 \/

dict :: Dict ((x \/ y) \/ z ⇒ x \/ y \/ z)
dict = Dict

GHC will blow up with the following error message:

ghc: panic! (the 'impossible' happened)
  (GHC version 8.10.4:
        GHC.StgToCmm.Env: variable not found
  irred_a1vk
  local binds for:
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1179:37 in ghc:Outputable
        pprPanic, called at compiler/GHC/StgToCmm/Env.hs:149:9 in ghc:GHC.StgToCmm.Env

Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug

Steps to reproduce

Compiling the module given above with GHC 8.10.4 should do it.

Expected behavior

GHC should not panic, and should either produce a type error or a successful compilation.

Environment

• GHC version used: 8.10.4