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

[Folkert de Vries via llvm-commits]

folkertdev wrote:

> > 
> 
> Those don't know about f16 libcalls, so those probably error if reached.

Is that a problem? should they know about the f16 libcalls? adding an extra argument to `GetFPLibCall` is going to be kind of invasive.

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