Fix a bug causing loss of sharing in `UniqSDFM`

While fixing #18610, I noticed that
```hs
f :: Bool -> Int
f x = case (x, x) of
  (True,  True)  -> 1
  (False, False) -> 2
```
was *not* detected as exhaustive. I tracked it down to `equateUSDFM`,
where upon merging equality classes of `x` and `y`, we failed to atually
indirect the *representative* `x'` of the equality class of `x` to the
representative `y'` of `y`.

The fixed code is much more naturally and would I should have written in
the first place. I can confirm that the above example now is detected as
exhaustive. The commit that fixes #18610 comes directly after and it has
`f` above as a regression test, so I saw no need to open a ticket or
commit a separate regression test.

1 files changed, 2 insertions, 2 deletions

diff --git a/compiler/GHC/Types/Unique/SDFM.hs b/compiler/GHC/Types/Unique/SDFM.hs
index a0871909ed..b34c4b3f94 100644
--- a/compiler/GHC/Types/Unique/SDFM.hs
+++ b/compiler/GHC/Types/Unique/SDFM.hs
@@ -88,8 +88,8 @@ equateUSDFM usdfm@(USDFM env) x y =
   case (lu x, lu y) of
     ((x', _)    , (y', _))
       | getUnique x' == getUnique y' -> (Nothing, usdfm) -- nothing to do
-    ((x', _)    , (_ , Nothing))     -> (Nothing, set_indirect y x')
-    ((_ , mb_ex), (y', _))           -> (mb_ex,   set_indirect x y')
+    ((x', _)    , (y', Nothing))     -> (Nothing, set_indirect y' x')
+    ((x', mb_ex), (y', _))           -> (mb_ex,   set_indirect x' y')
   where
     lu = lookupReprAndEntryUSDFM usdfm
     set_indirect a b = USDFM $ addToUDFM env a (Indirect b)