Speed up changesUponFlattening

Since the first Union alternative was produced by the previous
call to to changesUponFlattening, it seems unnecessary to
scrutinize it again.

This speeds up some prettyprinter-heavy applications in dhall
by 20 to 42%.

This assumes some kind of idempotence in changesUponFlattening.

Fixes quchen#99.

prettyprinter/src/Data/Text/Prettyprint/Doc/Internal.hs

@@ -553,7 +553,7 @@ changesUponFlattening :: Doc ann -> Maybe (Doc ann)
 changesUponFlattening = \doc -> case doc of
     FlatAlt _ y     -> Just (flatten y)
     Line            -> Just Fail
-    Union x _       -> changesUponFlattening x <|> Just x
+    Union x _       -> Just x
     Nest i x        -> fmap (Nest i) (changesUponFlattening x)
     Annotated ann x -> fmap (Annotated ann) (changesUponFlattening x)