[llvm] [AMDGPU][SIInsertWaitcnts][NFC] Drop IsExpertMode and MaxCounter member variables (PR #181092)

[via llvm-commits]

vporpo wrote:

>  isExpertMode function might have to look up function attributes by name, which is not what I would call trivial.

Yeah, even though most invocations are not in loops, I think there a few of them are, especially after merging with the AsyncMarks changes. So I will change this to keep IsExpertMode as a member variable.

https://github.com/llvm/llvm-project/pull/181092