target-arm: A64: Saturating and narrowing shift ops

This implements the remaining [US][Q][R]SHR[U][N][2] opcodes, which are
saturating and narrowing shift right operations. These are used in
things like libav. Note signed shifts can have an "unsigned" saturating
narrow operation which will floor negative values.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1394822294-14837-7-git-send-email-peter.maydell@linaro.org
[PMM: Added the scalar encodings, style tweaks]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
