accel/tcg: Improve can_do_io management

accel/tcg: Improve can_do_io management

We already attempted to set and clear can_do_io before the first
and last insns, but only used the initial value of max_insns and
the call to translator_io_start to find those insns.

Now that we track insn_start in DisasContextBase, and now that
we have emit_before_op, we can wait until we have finished
translation to identify the true first and last insns and emit
the sets of can_do_io at that time.

This fixes the case of a translation block which crossed a page
boundary, and for which the second page turned out to be mmio.
In this case we truncate the block, and the previous logic for
can_do_io could leave a block with a single insn with can_do_io
set to false, which would fail an assertion in cpu_io_recompile.

Reported-by: Jørgen Hansen <Jorgen.Hansen@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Jørgen Hansen <Jorgen.Hansen@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>