[llvm] Fix legalizing `FNEG` and `FABS` with `TypeSoftPromoteHalf` (PR #156343)

[via llvm-commits]

beetrees wrote:

`FCOPYSIGN` is [already correctly handled](https://github.com/llvm/llvm-project/blob/96e4caadb49e4810a70b477279bf12f205ff1431/llvm/lib/CodeGen/SelectionDAG/LegalizeFloatTypes.cpp#L3432-L3478).

https://github.com/llvm/llvm-project/pull/156343