target/arm: Split out disas_a64_legacy

target/arm: Split out disas_a64_legacy

Split out all of the decode stuff from aarch64_tr_translate_insn.
Call it disas_a64_legacy to indicate it will be replaced.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20230512144106.3608981-2-peter.maydell@linaro.org
[PMM: Rebased]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>