[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 think the windows test failure is spurious 

https://github.com/llvm/llvm-project/pull/171904#issuecomment-3643887634

if so, I think this is ready.

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