[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:

Libcall is a mechanical property, it doesn't change the type or rounding issues. The mixing of the libcall emission into type legalization for the soft float case is an unfortunate DAG-ism 

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