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

[Sameer Sahasrabuddhe via llvm-commits]

ssahasra wrote:

Calling SIInsertWaitcnts the "parent" of WaitcntBrackets smells wrong. We could at least call it something else, like "Context"? Or capture it as a separate Context structure and pass that to WaitcntBrackets?

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