Fix quadratic behavior of prepareAlts

Summary:
This code is quadratic and a simple test case I used
managed to tickle it.

The example (same one as #14667) looks like this:
```
module A10000 where

 data A = A
   | A00001
   | A00002
 ...
   | A10000

 f :: A -> Int
 f A00001 = 19900001
 f A00002 = 19900002
 ...
 f A10000 = 19910000
```

Applied on top of a fix for #14667, it gives a 30% compile time
improvement.

Test Plan: ./validate

Reviewers: simonpj, bgamari

Subscribers: rwbarton, thomie, simonmar, carter

Differential Revision: https://phabricator.haskell.org/D4307

0 files changed, 0 insertions, 0 deletions