[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:

> In any case even promoting to f64 will not give accurate results for bf16

That surprises me, really? If we don't already have tests relying on it, I'd probably return poison and emitError 

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