TH-spliced class instances are pretty-printed incorrectly post-#3384

The HsSyn prettyprinter tests patch 499e4382 broke the pretty-printing of Template Haskell-spliced class instances. You can see this for yourself by compiling this code with GHC HEAD:

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE TemplateHaskell #-}
{-# OPTIONS_GHC -ddump-splices #-}
module Bug where

class C a b

$([d| instance C a (Maybe b)
    |])

$ ~/Software/ghc3/inplace/bin/ghc-stage2 --interactive Bug.hs
GHCi, version 8.1.20170126: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )
Bug.hs:(9,3)-(10,6): Splicing declarations
    [d| instance C a_a1eC (Maybe b_a1eD) |]
  ======>
    instance C a_a4m5 Maybe b_a4m6

Note the nonsensical instance C a_a4m5 Maybe b_a4m6. For comparison, here is how it should be pretty-printed:

$ /opt/ghc/8.0.2/bin/ghci Bug.hs
GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )
Bug.hs:(9,3)-(10,6): Splicing declarations
    [d| instance C a_a13N (Maybe b_a13O) |]
  ======>
    instance C a_a3Ju (Maybe b_a3Jv)

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	alanz
Operating system
Architecture