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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> 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

We currently have both approaches in TargetPassConfig, which makes it difficult to follow where passes come from. We have addPass/insertPass/disablePass/substitutePass based on PassID, which are really hard to understand. We additionally have the assorted pipeline insertion point callbacks.

We actually call all the insertPass stuff inside the pipeline callbacks, but I don't think the point you call these actually matters which adds to the confusion. Overall I think it would be better to just have the assorted insertion point callbacks 

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