-O1 changes result at runtime, duplicating __DEFAULT case

Here is a program, which works as expected in GHC 8.4.1-alpha3 with -O0, but changes it behaviour with -O1.

{-# LANGUAGE MagicHash #-}

import qualified Data.Vector.Unboxed as U
import GHC.Exts

vec :: U.Vector Moebius
vec = U.singleton Moebius0

main :: IO ()
main = print $ U.head vec == U.head vec

data Moebius = Moebius0 | Moebius1 | Moebius2
  deriving (Eq)

fromMoebius :: Moebius -> Int
fromMoebius Moebius0 = 0
fromMoebius Moebius1 = 1
fromMoebius Moebius2 = 2

toMoebius :: Int -> Moebius
toMoebius (I# i#) = tagToEnum# i#

{- ...unboxed vector instances, see file attached... -}

It is expected that this program will print True. However, when compiled with -O1 it prints False.

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.4.0.20180204
$ ghc -O0 Sieve.hs && ./Sieve
[1 of 1] Compiling Main             ( Sieve.hs, Sieve.o ) [Optimisation flags changed]
Linking Sieve ...
True
$ ghc -O1 Sieve.hs && ./Sieve
[1 of 1] Compiling Main             ( Sieve.hs, Sieve.o ) [Optimisation flags changed]
Linking Sieve ...
False

It reproduces on OS X and Ubuntu, but worked fine in GHC 8.2.

I looked into generated Core and found a suspicious function, having two __DEFAULT cases with different bodies.

main2 :: String
main2
  = case vec `cast` <Co:3> of
    { Vector ipv_sb7L ipv1_sb7M ipv2_sb7N ->
    case <# 0# ipv1_sb7M of {
      __DEFAULT -> case main3 ipv1_sb7M of wild_00 { };
      1# ->
        case indexIntArray# ipv2_sb7N ipv_sb7L of {
          __DEFAULT -> $fShowBool4;
          __DEFAULT -> $fShowBool2;
          1# -> $fShowBool2;
          2# -> $fShowBool2
        }
    }
    }

Trac metadata

Trac field	Value
Version	8.4.1-alpha3
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler
Test case
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