target/i386: [tcg] Port to translate_insn

Incrementally paves the way towards using the generic instruction translation
loop.

Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Emilio G. Cota <cota@braap.org>
Message-Id: <150002195074.22386.16195894320027075398.stgit@frigg.lan>
Signed-off-by: Richard Henderson <rth@twiddle.net>
