[llvm] [AMDGPU] Mark ASYNCMARK as meta instruction to fix hazard cycle miscounting (PR #189981)

[Ryan Mitchell via llvm-commits]

RyanRio wrote:

> TSFlag bits are precious and it's not worth wasting one on a single instruction case. Just add the helper function

Good point, oh well

https://github.com/llvm/llvm-project/pull/189981