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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> ExpandFloatRes_FMA and SoftenFloatRes_FMA already emit libcalls, so they don't seem relevant. 

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

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