[llvm] [GlobalISel] Fall back for bf16 conversions. (PR #71470)

[David Green via llvm-commits]

davemgreen wrote:

> > It was previously giving an error in the frontend that you are using an operation that is not supported, but that was broken at some point without making the backend work correctly.
> 
> Is this the kind of error?

Yep, that's the error. Operations like Add were previously not defined as they were not supported by the instruction set and there were no promotions for them. Someone changed how the frontend worked whilst only making the backend work for X86.

https://github.com/llvm/llvm-project/pull/71470