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

[Folkert de Vries via llvm-commits]

folkertdev wrote:

> Are backports still being accepted for the current release branch?

They are but even on x86_64 there is a lot of churn and the patch (if I remember right) doesn't apply cleanly even on a `main` checkout from a week ago, much less the 23.x branch.

So I'd like to backport if of course but I think that'll be a tough sell.

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