[clang] [llvm] [AMDGPU] Use `bf16` instead of `i16` for bfloat (PR #80908)

[Stanislav Mekhanoshin via cfe-commits]

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

Thanks. There are definitely at least 2 outstanding problems, but it seems there are no regressions comparing to what we have now. LGTM.

https://github.com/llvm/llvm-project/pull/80908