Fix as-pattern for GHC 9.

[rumkeller]

Fixes the following error:

src/LLVM/Pretty.hs:478:7: error:
    Found a binding for the ‘@’ operator in a pattern position.
    Perhaps you meant an as-pattern, which must not be surrounded by whitespace
    |
478 |     e @ Invoke {..} ->

[edgar-gip]

Ah, I was going to prepare a pull request with the same change since I noticed the breakage, but now I see this has been sitting for two years.

It's too bad, since it prevents using this package with modern versions of GHC, and it's a trivial and safe fix...