Summary:
Previously, one could experience an error message like this:

```
Expected: T (a -> Either a b)
  Actual: T (a -> Either a b)
```

This makes the error message an iota clearer by tidying it first, which will
instead produce:

```
Expected: T (a1 -> Either a1 b1)
  Actual: T (a -> Either a b)
```

Which steers users towards the understanding that the two sets of tyvars are
actually different.

Test Plan: make test TEST=T13972

Reviewers: simonpj, austin, bgamari, goldfire

Reviewed By: goldfire

Subscribers: goldfire, rwbarton, thomie

GHC Trac Issues: #13972

Differential Revision: https://phabricator.haskell.org/D3820