The muladd (inf,zero,nan) case sets InvalidOp and returns the
input value 'c', and prefer sNaN over qNaN, in c,a,b order.
Binary operations prefer sNaN over qNaN and a,b order.

Signed-off-by: Song Gao <gaosong@loongson.cn>
Message-Id: <20220716085426.3098060-3-gaosong@loongson.cn>
[rth: Add specialization for pickNaN]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>