[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.
> 
> Yes. I want tablegen to start generating all the get* libcall helper functions to assist this

Sure, anything I should do here now though?

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