As we will slowly move to decodetree generated decoders,
extract the legacy decoding from decode_opc(), so new
decoders are added in decode_opc() while old code is
removed from decode_opc_legacy().

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20201215225757.764263-2-f4bug@amsat.org>