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

[Matt Arsenault via llvm-commits]

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arsenm wrote:

```suggestion
  %res = call double @llvm.fma.f64(double %f1, double %f2, double %acc)
```

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