[llvm] [NFC] Add useFPRegsForHalfType(). (PR #74147)

[Krzysztof Parzyszek via llvm-commits]

kparzysz wrote:

There are a handful of functions there that assume `MVT::i16` for representing `half` values, e.g. https://github.com/llvm/llvm-project/blob/main/llvm/lib/CodeGen/SelectionDAG/LegalizeFloatTypes.cpp#L2806-L2812


https://github.com/llvm/llvm-project/pull/74147