[llvm] [AMDGPU] Mark WAIT_ASYNCMARK as zero-size instruction (PR #195107)

[Zhewen Yu via llvm-commits]

Yu-Zhewen wrote:

Added a test. Happy to also add generic size skipping for meta instructions in `getInstSizeInBytes()` if you'd prefer, but I'm not familiar with the context here

https://github.com/llvm/llvm-project/pull/195107