[llvm] AMDGPU/GFX12: Do not wait unnecessarily before barriers (PR #154970)

[Jay Foad via llvm-commits]

jayfoad wrote:

We could get the same effect by saying GFX12 has FeatureBackOffBarrier, right? But perhaps your way is more honest.

I have a slight concern that this could trigger the same kind of problems we have seen on GFX10 and GFX11, which we worked around by removing FeatureBackOffBarrier from them. But if not then this seems fine to me.

https://github.com/llvm/llvm-project/pull/154970