[llvm] Add PR check to suggest alternatives to using undef (PR #118506)

[David Spickett via llvm-commits]

DavidSpickett wrote:

That looks much better.

But I wonder if you will need to expand the comment if for example folks (like me :) ) have little idea what undef and poison are in the first place. So if you put this somewhere in the documentation and linked to it from the comment, you can update the documentation and everyone benefits. Like:
```
See [this documentation](link) for more explanation and alternatives to using undef."
```
And for anyone who starts with the documentation, they will be aware of how (not) to use undef as well.

Maybe it could go in the langref as part of the description of undef?

https://github.com/llvm/llvm-project/pull/118506