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

[Jay Foad via llvm-commits]

jayfoad wrote:

> Calling SIInsertWaitcnts the "parent" of WaitcntBrackets smells wrong. We could at least call it something else, like "Context"?

I guess "Context" would be OK, but it still seems a bit vague to me. How about "Pass"?

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