nir_type_conversion enables new operations to handle rounding modes to
convert to fp16 values. Two new opcodes are enabled nir_op_f2f16_rtne
and nir_op_f2f16_rtz.

The undefined behaviour doesn't has any effect and uses the original
nir_op_f2f16 operation.

v2: Indentation fixed (Jason Ekstrand)

v3: Use explicit case for undefined rounding and assert if
    rounding mode is used for non 16-bit float conversions
    (Jason Ekstrand)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>