This fixes a non-determinism bug where where depending on the
order of uniques allocated, the specialized workers would have different
order of arguments.

Compare:

```
  $s$wgo_s1CN :: Int# -> Int -> Int#
  [LclId, Arity=2, Str=DmdType <L,U><L,U>]
  $s$wgo_s1CN =
    \ (sc_s1CI :: Int#) (sc_s1CJ :: Int) ->
      case tagToEnum# @ Bool (<=# sc_s1CI 0#) of _ [Occ=Dead] {
        False ->
          $wgo_s1BU (Just @ Int (I# (-# sc_s1CI 1#))) (Just @ Int sc_s1CJ);
        True -> 0#
      }
```

vs

```
  $s$wgo_s18mTj :: Int -> Int# -> Int#
  [LclId, Arity=2, Str=DmdType <L,U><L,U>]
  $s$wgo_s18mTj =
    \ (sc_s18mTn :: Int) (sc_s18mTo :: Int#) ->
      case tagToEnum# @ Bool (<=# sc_s18mTo 0#) of _ [Occ=Dead] {
        False ->
          $wgo_s18mUc
            (Just @ Int (I# (-# sc_s18mTo 1#))) (Just @ Int sc_s18mTn);
        True -> 0#
      }
```

Test Plan:
I've added a new testcase
./validate

Reviewers: simonmar, simonpj, austin, goldfire, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1508

GHC Trac Issues: #4012