[llvm] [X86] Make bf16 a real register-class type (PR #214487)

[via llvm-commits]

tfzee wrote:

> TBH I had just expected a trivial peephole to fix #155270 - I really don't see the point of much of this

Sure, I thought this might decrease the amount of dedicated peephole fixes required long term and also saw someone started it before.  
I'm gonna make a PR #218359 with a custom lowering instead that seems like the simplest fix targeting this issue in particular.
Maybe it still makes sense to add parts of the tests since bf16 does not have many right now. 

https://github.com/llvm/llvm-project/pull/214487