[llvm] [ExpandVariadics] Move NewPM class to RequiredPassInfoMixin (PR #216597)

[Alex MacLean via llvm-commits]

AlexMaclean wrote:

> LGTM.
> 
> Although I'm not sure this creates a functional difference given this is a module pass.

I believe this will prevent `-opt-bisect-limit` from disabling this pass. 

https://github.com/llvm/llvm-project/pull/216597