Clean vp9_highbd_iht4x4_16_add_neon()
Clean vp9_highbd_iht4x4_16_add_neon()

Extract common code.

Change-Id: I422150ada1c6915f0ce39b912149994eb3bb3f12

webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx