aco: always use rtne for fquantize2f16
The SPIR-V spec says:
If Value is positive with a magnitude too large to represent as a 16-bit floating-point value, the result is positive infinity. If Value is negative with a magnitude too large to represent as a 16-bit floating-point value, the result is negative infinity.
This is only the case for rtne v_cvt_f16_f32