[llvm] [AMDGPU] Move common fields out of WaitcntBrackets. NFC. (PR #148864)

[Stephen Thomas via llvm-commits]

stepthomas wrote:

I like it in principle, not just for the reasons listed but because it feels like it will make changes that require refactoring across interfaces much less annoying. I agree that `Parent` is the wrong term, though - I'd also prefer `Context`.

https://github.com/llvm/llvm-project/pull/148864