target/arm: Split out thumb_tr_translate_insn

We need not check for ARM vs Thumb state in order to dispatch
disassembly of every instruction.

Tested-by: Emilio G. Cota <cota@braap.org>
Reviewed-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>
