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.

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