PatternSynonms and DisambiguateRecordFields causes panic

The following module causes a panic in HEAD:

{-# LANGUAGE PatternSynonyms, DisambiguateRecordFields #-}
data P a = MkP a
pattern S{x} = MkP x
e = S{x = 3}

ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 7.11.20151223 for x86_64-unknown-linux):
	find_tycon
  S
  [S pattern synonym defined at PatSynBug.hs:3:1]

This is essentially the same bug as #9975 (closed); for some reason it became slightly harder to tickle. Fix incoming.

Trac metadata

Trac field	Value
Version	7.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture