softfloat: Share code between parts_pick_nan cases

softfloat: Share code between parts_pick_nan cases

Remember if there was an SNaN, and use that to simplify
float_2nan_prop_s_{ab,ba} to only the snan component.
Then, fall through to the corresponding
float_2nan_prop_{ab,ba} case to handle any remaining
nans, which must be quiet.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20241203203949.483774-10-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>