target/arm: Convert Neon VTRN to decodetree

target/arm: Convert Neon VTRN to decodetree

Convert the Neon VTRN insn to decodetree. This is the last insn in the
Neon data-processing group, so we can remove all the now-unused old
decoder framework.

It's possible that there's a more efficient implementation of
VTRN, but for this conversion we just copy the existing approach.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200616170844.13318-21-peter.maydell@linaro.org