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

[Folkert de Vries via llvm-commits]

https://github.com/folkertdev commented:

I'll need to update a bunch of tests (CI will tell me which), but I think the implementation should now cover all targets. 

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