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

[Harald van Dijk via llvm-commits]

hvdijk wrote:

An example of how this can be used, which I plan to submit after this: https://github.com/hvdijk/llvm-project/commit/softpromotehalf-fpreg-arm

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