target/arm: Convert Neon narrowing shifts with op==9 to decodetree

Convert the remaining Neon narrowing shifts to decodetree:
  * VQSHRN
  * VQRSHRN

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200522145520.6778-7-peter.maydell@linaro.org