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

Currently we generally use `i16` to represent `bf16` in those tablegen
files. This patch is trying to use `bf16` directly.

Fix #79369.

GitOrigin-RevId: 46734aa1e5fed0c6a11855280d83c3513b794aaa
1 file changed