aco: fix non-rtz pack_half_2x16
We were using the wrong conversion opcode. The high bits are also not zero'd on GFX10, which can cause v_cvt_pk_u16_u32 to clamp.
We were using the wrong conversion opcode. The high bits are also not zero'd on GFX10, which can cause v_cvt_pk_u16_u32 to clamp.