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

[Matt Arsenault via llvm-commits]

arsenm 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? 

No, that error doesn't make much sense 


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