[llvm] [AMDGPU] Handle expert scheduling VA_VDST WAR hazards (PR #201619)

[Jay Foad via llvm-commits]

jayfoad wrote:

Just to emphasise that VA_VDST_RD and VA_VDST_WR refer to the same hardware counter, I think we could put an assert in `WaitcntBrackets::merge` (or any other suitable place for enforcing invariants) that the LBs and UBs of these two counters are identical.

https://github.com/llvm/llvm-project/pull/201619