This adds 2-src variable shift register instructions:
C5.6.115 LSLV, C5.6.118 LSRV, C5.6.17 ASRV, C5.6.154 RORV

Signed-off-by: Alexander Graf <agraf@suse.de>
[claudio: adapted to new decoder, use enums for shift types]
Signed-off-by: Claudio Fontana <claudio.fontana@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>