A flawed test lead to the instructions always being treated as
unallocated encodings.

Fixes: https://bugs.launchpad.net/bugs/1813460Signed-off-by: NRemi Denis-Courmont <remi.denis.courmont@huawei.com>
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>