I_S tables were materialized too late, an attempt to use table
statistics before the table was created caused a crash.

Let's move table creation up. it only needs read_set to
be calculated properly, this happens in JOIN::optimize_inner(),
after semijoin transformation.

Note that tables are not populated at that point, so most of the
statistics would make no sense anyway. But at least field sizes
will be correct. And it won't crash.