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

Currently, half operations can be promoted in one of two ways.

* If softPromoteHalfType() returns false, fp16 values are passed around
in fp32 registers, and whole chains of fp16 operations are promoted to
fp32 in one go.
* If softPromoteHalfType() returns true, fp16 values are passed around
in i16 registers, and individual fp16 operations are promoted to fp32
and the result truncated to fp16 right away.

The softPromoteHalfType behavior is necessary for correctness, but
changing this for an existing target breaks the ABI. Therefore, this
commit adds a third option:

* If softPromoteHalfType() returns true and useFPRegsForHalfType()
returns true as well, fp16 values are passed around in fp32 registers,
but individual fp16 operations are promoted to fp32 and the result
truncated to fp16 right away.

This change does not yet update any target to make use of it.