[llvm] [NewPM][CodeGen] Add callback style codegen pass builder (PR #108690)

[Christudasan Devadasan via llvm-commits]

cdevadas wrote:

@paperchalice are you planning to port Greedy allocator?
If you haven't started, let us know, we're planning for it.

https://github.com/llvm/llvm-project/pull/108690