[llvm] [AMDGPU] Reenable BackOffBarrier on GFX11/12 (PR #155370)

[Jay Foad via llvm-commits]

jayfoad wrote:

> Re-enable it by adding a wait on vm_vsrc before every barrier "start" instruction.
> This is a less strong wait than what we do without BackOffBarrier,

Description should mention that we only add the wait in CU mode? Is that why you describe it as "less strong" than without BackOffBarrier?

https://github.com/llvm/llvm-project/pull/155370