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

[Krzysztof Parzyszek via llvm-commits]

kparzysz wrote:

> Inside functions, I continue to use `MVT::i16` for representing `half` values.

Are you trying to only apply this promotion to function arguments and return values?  If so, then it should be entirely done in handling a calling convention.  This PR changes type legalization which will apply to all values of type `MVT::fp16`.

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