Fix errors in the decoding of ARM VQSHL/VQSHLU immediate forms,
including using the new VQSHLU helper functions where appropriate.

Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>