[AMDGPU] Tweak VOP3_INTERP16 profile

Set the output register class based on the output type, instead of
hard-coding VGPR_32. I think this is more correct. It doesn't make any
difference at the moment because we use the same class for 16- and
32-bit results, but it might in future if we make more use of true
16-bit register classes.

Differential Revision: https://reviews.llvm.org/D102622