target/i386: [tcg] Port to insn_start

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

Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
Reviewed-by: Emilio G. Cota <cota@braap.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Alex Benneé <alex.benee@linaro.org>
Message-Id: <150002146647.22386.13380064201042141261.stgit@frigg.lan>
Signed-off-by: Richard Henderson <rth@twiddle.net>
