[llvm] fix `llvm.fma.f16` double rounding issue when there is no native support (PR #171904)

[Folkert de Vries via llvm-commits]

================

----------------
folkertdev wrote:

I'm not sure, based on https://github.com/llvm/llvm-project/issues/131531 the `bf16` variant should be a libcall unconditionally.

https://github.com/llvm/llvm-project/pull/171904