GHC 8.4.1-alpha panics when optimizing function using getTag and tagToEnum#

I originally observed this panic in this Travis build. The panic can be reduced to this file:

{-# LANGUAGE MagicHash #-}
{-# LANGUAGE TypeFamilies #-}
module Bug where

import GHC.Base (getTag)
import GHC.Exts (Int(..), tagToEnum#)

data family TyFamilyEnum
data instance TyFamilyEnum = TyFamilyEnum1 | TyFamilyEnum2 | TyFamilyEnum3

suc :: TyFamilyEnum -> TyFamilyEnum
suc a_aaf8
        = case getTag a_aaf8 of
             a_aaf9
               -> if 2 ==  I# a_aaf9
                  then error "succ{TyFamilyEnum}: tried to take `succ' of last tag in enumeration"
                  else case I# a_aaf9 + 1 of
                         I# i_aafa -> tagToEnum# i_aafa :: TyFamilyEnum

In GHC 8.2.2, compiling this with optimization works fine. But in GHC 8.4.1-alpha, it panics:

$ /opt/ghc/8.4.1/bin/ghc -fforce-recomp -O1 Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.4.0.20171222 for x86_64-unknown-linux):
        Prelude.!!: index too large

Trac metadata

Trac field	Value
Version	8.4.1-alpha1
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler
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