[mlir][AMDGPU] Remove an old bf16 workaround (#108409)
The AMDGPU backend now implements LLVM's `bfloat` type. Therefore, we no
longer need to type convert MLIR's `bf16` to `i16` during lowerings to
ROCDL.
As a result of this change, we discovered that, whel the code for MFMA
and WMMA intrinsics was mainly prepared for this change, we were failing
to bitcast the bf16 results of WMMA operations out from the i16 they're
natively represented as. This commit also fixes that issue.
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Co-authored-by: Jakub Kuderski <kubakuderski@gmail.com>
