[llvm] [RFC][AMDGPU][InsertWaitCnt] Move WaitEventType into separate HWEvent header (PR #202886)

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

Note for reviewers: I am not strongly attached to these patches. I am just using some spare cycles to try to declutter InsertWaitCnt a bit. I am okay if the consensus is that this is not a good idea :sunglasses: 

https://github.com/llvm/llvm-project/pull/202886