[llvm] [AMDGPU] Add wait states between different MFMAs sharing an accumulator (PR #218363)

[Christudasan Devadasan via llvm-commits]

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

LGTM. Wait for a day or so for others. @arsenm had some concerns earlier about the test coverage.

https://github.com/llvm/llvm-project/pull/218363