Fix Trac #3591: very tricky specialiser bug

  
There was a subtle bug in the interation of specialisation and floating,
described in Note [Specialisation of dictionary functions]. 
  
The net effect was to create a loop where none existed before; plain wrong.
  
In fixing it, I did quite a bit of house-cleaning in the specialiser, and
added a lot more comments.  It's tricky, alas.

compiler/basicTypes/Id.lhs

@@ -49,7 +49,7 @@ module Id (
 	isImplicitId, isDeadBinder, isDictId, isStrictId,
 	isExportedId, isLocalId, isGlobalId,
 	isRecordSelector, isNaughtyRecordSelector,
-	isClassOpId_maybe,
+	isClassOpId_maybe, isDFunId,
 	isPrimOpId, isPrimOpId_maybe, 
 	isFCallId, isFCallId_maybe,
 	isDataConWorkId, isDataConWorkId_maybe, isDataConId_maybe, idDataCon,
@@ -326,6 +326,7 @@ isNaughtyRecordSelector :: Id -> Bool
 isPrimOpId              :: Id -> Bool
 isFCallId               :: Id -> Bool
 isDataConWorkId         :: Id -> Bool
+isDFunId                :: Id -> Bool
 
 isClassOpId_maybe       :: Id -> Maybe Class
 isPrimOpId_maybe        :: Id -> Maybe PrimOp
@@ -348,6 +349,10 @@ isPrimOpId id = case Var.idDetails id of
                         PrimOpId _ -> True
                         _          -> False
 
+isDFunId id = case Var.idDetails id of
+                        DFunId -> True
+                        _      -> False
+
 isPrimOpId_maybe id = case Var.idDetails id of
                         PrimOpId op -> Just op
                         _           -> Nothing