[llvm] [NewPM][CodeGen] Refactoring CodeGenPassBuilder (PR #89708)

[Arthur Eubanks via llvm-commits]

aeubanks wrote:

sorry for the slow review on this, I've been trying to find time to write down my thoughts and understand the current codegen pipeline some more

all this removing and replacing of passes from a "default" codegen pipeline makes me nervous, especially with all the templatey stuff, plus being able to choose e.g. the third run of some pass. I'd really like to understand if we can do the "building blocks" approach above, perhaps with the help of some callbacks for targets to add passes into certain portions of the pipeline. IMO that seems more preferable and more understandable than a default pipeline that's completely customizable at every pass, which is hard to follow and doesn't seem principled

@arsenm thoughts?

https://github.com/llvm/llvm-project/pull/89708