tcg: Hoist max_insns computation to tb_gen_code

tcg: Hoist max_insns computation to tb_gen_code

In order to handle TB's that translate to too much code, we
need to place the control of the length of the translation
in the hands of the code gen master loop.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>