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

[via llvm-commits]

tfzee wrote:

@RKSimon could you take a quick look if this is viable route in your opinion. (Don't need full review yet)

It extends #134859 to work correctly for all the testcases I came up with(mostly mirroring the f16 testcases).
The PR got quite big because I didn't want to introduce any regressions where possible.(Its mostly test additions aswell)
The Issue #155270 could also be fixed easier by custom lowering but it feels like making it a legal type and handling it similar to other types is cleaner in the long term compared to handling all the cases as custom lowering. 


Would be my first big PR to LLVM so if I should do anything differently open for suggestions/feedback :) 

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