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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Nice. I was thinking of adding this check for aarch64 at least, it makes sense to do it generically.
> * to have it as part of the type information (bf16 as opposed to s16)

We were talking about adding it to LLT. I think we should steal some bits from the size or element count and turn it into an enum for the class, such that any FP type would still be isScalar but identifiable 

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