[llvm] [AMDGPU] BackOffBarrier feature added to gfx1250; Removed incorrect "DS Store drain" check. (PR #179818)

[via llvm-commits]

hidekisaito wrote:

>Maybe invert it to CannotBackOffBarrier, so that only the old architectures that need the "workaround" of adding a wait before a barrier need the feature?

If we'd want to do this, that should be a separate PR. Are there any other concerns w.r.t. the current PR?

https://github.com/llvm/llvm-project/pull/179818