[llvm] [RFC][AMDGPU][InsertWaitcnts] Move `WaitcntBrackets` to a separate file (PR #210044)

[Jay Foad via llvm-commits]

jayfoad wrote:

> While I think this is a net positive, I don't want to force this through if others feel it's unnecessary.

My feelings are pretty much the opposite: I'm unsure of the benefit, and would find the churn mildly annoying, mostly because I no longer know which source files to open to find all different parts of waitcnt insertion, but I don't want to block this if others are in favour.

https://github.com/llvm/llvm-project/pull/210044