target/arm: Convert Neon VSHL and VSLI 2-reg-shift insn to decodetree

Convert the VSHL and VSLI insns from the Neon 2-registers-and-a-shift
group to decodetree.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200522145520.6778-2-peter.maydell@linaro.org