[llvm] [AMDGPU][SIInsertWaitcnts][NFC] Simplify logic in GFX12Plus::applyPreexistingWaitcnts (PR #184925)

[Ryan Mitchell via llvm-commits]

https://github.com/RyanRio approved this pull request.

That UpdatableInstr has always been a thorn in my brain, thanks. Approved on clean CI.

https://github.com/llvm/llvm-project/pull/184925