gen_arith_imm() does a lot of decoding manually, which was hard to read
in case of the shift instructions and is not necessary anymore with
decodetree.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Signed-off-by: Peer Adelt <peer.adelt@hni.uni-paderborn.de>