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

[Folkert de Vries via llvm-commits]

folkertdev wrote:

> This kind of looks backwards - I would have expected the default type of a bf16 to be like a f32

`i16` seems to be a better default for targets that have limited support, defaulting to `f32` causes the issues with bitcasting that this PR is meant to avoid.

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