[llvm] [CodeGen] computeRegisterProperties - fix `bf16` to `i16` bitcast quieting NaNs (PR #224867)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

please update the summary - we don't follow the f16 pattern anymore (and in fact we're doing what ARM should be doing for f16).

@arsenm any comments on the amdgpu chages?

https://github.com/llvm/llvm-project/pull/224867