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

[Jay Foad via llvm-commits]

jayfoad wrote:

Really I'd like to make `WaitcntBrackets` a Java-style **inner class** of `SIInsertWaitcnts`, so that every instance of `WaitcntBrackets` can implicitly access an enclosing _instance_ of `SIInsertWaitcnts`.

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