target/arm: Merge disas_a64_insn into aarch64_tr_translate_insn

target/arm: Merge disas_a64_insn into aarch64_tr_translate_insn

It is confusing to have different exits from translation
for various conditions in separate functions.

Merge disas_a64_insn into its only caller.  Standardize
on the "s" name for the DisasContext, as the code from
disas_a64_insn had more instances.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210821195958.41312-3-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>