[llvm] [AMDGPU][SIInsertWaitcnts] drop OldWaitcntInstr only when it is processed (PR #145720)

[Juan Manuel Martinez Caamaño via llvm-commits]

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

Thanks! Everything else looks good to me.

https://github.com/llvm/llvm-project/pull/145720