How to use the PassManagerBuilder & adjustPassManager

Many target machines, like AMDGPU and NVPTX, have target-specific passes that should be added at various points int the optimization chain. The current interface is a TargetMachine `adjustPassManager` API to be used with a PassManagerBuilder, which we don't use. I'm not sure how we should use to using one, as we can't create an instance with our own passes and extension points -- we can only work with LLVM's.

Start by examining the TargetMachine adjustPassManager API and PassManagerBuilder integration described in the issue, focusing on how target-specific passes enter the optimization chain. The issue names no files or tests; done requires a documented or implemented approach agreed with maintainers.

Written by the indexing model from the issue text.