The idea is that instead of marking all select_lex's with DISTINCT, we
only mark those that really need distinct result.

Benefits of this change:
- Temporary tables used with derived tables, UNION, IN are now smaller
  as duplicates are removed already on the insert phase.
- The optimizer can now produce better plans with EQ_REF. This can be
  seen from the tests where several queries does not anymore materialize
  derived tables twice.
- Queries affected by 'in_predicate_conversion_threshold' where large IN
  lists are converted to sub query produces better plans.

Other things:
- Removed on duplicate call to sel->init_select() in
  LEX::add_primary_to_query_expression_body()
- I moved the testing of
  tab->table->pos_in_table_list->is_materialized_derived()
  in join_read_const_table() to the caller as it caused problems for
  derived tables that could be proven to be const tables.
  This also is likely to fix some bugs as if join_read_const_table()
  was aborted, the table was left marked as JT_CONST, which cannot
  be good.  I added an ASSERT there for now that can be removed when
  the code has been properly tested.