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

[via llvm-commits]

erickq 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? https://godbolt.org/z/v8d4foaef
![image](https://github.com/llvm/llvm-project/assets/12108571/27e253b5-4280-4789-a730-62234dd0ec47)


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