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

[Folkert de Vries via llvm-commits]

folkertdev wrote:

> We are allowed to create manual cherry picks for PRs into a release branch, this patch wouldn't be that much work.

Right, true.

> At this stage you might have to demonstrate a notable regression in 23.x vs 22.x

Right, also a tough sell. I think our actual motivation is to be able to use this in rust 4 months sooner (and possibly find further issues early). On the other hand the blast radius of this change seems small, and clearly `bfloat` is kind of broken already on the affected targets.

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Anyway, let's actually merge this first and then we'll see.

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