[llvm] [DeveloperPolicy] Add guidelines for adding/enabling passes (PR #158591)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

Thanks for putting this up! This matches my expectations, and I think is in line with current practice for enabling new passes in the last few years.

https://github.com/llvm/llvm-project/pull/158591