[llvm] [AMDGPU][SIInsertWaitcnts][NFC] Drop `AMDGPU::` (PR #180663)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> A previous PR #178345 introduced `using namespace llvm::AMDGPU`, so this patch drops `AMDGPU::`.

I'd probably go for dropping the using namespace 

https://github.com/llvm/llvm-project/pull/180663