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

[via llvm-commits]

erickq wrote:

> I don't know of anyone working on it, but it would be good to get it fixed.

Hello, can you help me? Thank you very much. Are the 16-bit values obtained from `fcvt h0, s0` and `bl __truncsfbf2` equivalent?

https://godbolt.org/z/q5G8PEMz6 @davemgreen 

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