[llvm] [PowerPC] Legalize scalar bfloat operations and conversions (PR #221536)

[Aman Maurya via llvm-commits]

amanmaurya92 wrote:

I eliminated all by-value bfloat function arguments and return values from the tests.
Once all CI are green, I think this pr is complete.

@RolandF77 I am very interested in helping close this 16-bit float ABI gap (`_Float16` / `bfloat16`) in the PowerPC backend. 
Is there an active RFC, an LLVM Discourse thread that I could look into or help implement? I would love to contribute to this area.

https://github.com/llvm/llvm-project/pull/221536