[llvm] [Codegen][NewPM] Explicitly Nest Passes in CodegenPassBuilder (PR #169867)

[Arthur Eubanks via llvm-commits]

https://github.com/aeubanks commented:

I think this is an improvement, thanks

it doesn't make sense that we pass around MPM but not FPM/MFPM, I think we should be consistent about that. perhaps we put the MPM/FPM/MFPM in a helper impl struct to discourage people from accidentally bypassing the `add*Pass` APIs and directly adding to the PM 

https://github.com/llvm/llvm-project/pull/169867