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

[Harald van Dijk via llvm-commits]

hvdijk wrote:

So, revisiting this... The `TransformToType` has two uses. Both are for what type to legalize to, but at different times. There is the type legalization of function parameters and return values, which bypasses the calling convention so is tricky to handle elsewhere. And there is the type legalization for internal values within functions. However, for `TypeSoftPromoteHalf`, `TransformToType` is almost entirely ignored for internal values within functions, most `TypeSoftPromoteHalf` always promotes to `i16`, and that is fine, there is no requirement for type legalization to actually legalize to the type set in `TransformToType`. If I go over the lot and ensure all promotion is to `i16`, would that then make this PR acceptable?

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