ppc/translate: Delay NaN checking after comparison

Since we always perform a comparison between the two operands avoid
checking for NaN unless the result states they're unordered.

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Giuseppe Musacchio <thatlemon@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20201112230130.65262-4-thatlemon@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>