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

[Ulrich Weigand via llvm-commits]

uweigand wrote:

> I made the changes to `SoftPromoteHalfRes_FMAD`, and they help in some cases, but not in all of them. I'm missing something, and I'm not sure what that would be.

For SystemZ, we're not using `SoftPromoteHalf`, but a regular `Promote` operation.
 
> `ExpandFloatRes_FMA` and `SoftenFloatRes_FMA` already emit libcalls, so they don't seem relevant. Changing `PromoteFloatRes_FMAD` does not seem to have an effect.

That's where I thought we should end up ...


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