In a subselect all fields from outer selects are marked as dependent on
selects they are belong to. In some cases optimizer substitutes it for an
equivalent expression. For example "a_field IN (SELECT outer_field)" is
substituted with "a_field = outer_field". As we moved the outer_field to the
upper select it's not really outer anymore. But it was left marked as outer.
If exists an index over a_field optimizer choose wrong execution plan and thus
return wrong result.

Now the Item_in_subselect::single_value_transformer function removes dependent
marking from fields when a subselect is optimized away.

mysql-test/r/subselect.result:
  Added a test case for the bug#46051.
mysql-test/t/subselect.test:
  Added a test case for the bug#46051.
sql/item_subselect.cc:
  Bug#46051: Incorrectly market field caused wrong result.
  Now the Item_in_subselect::single_value_transformer function removes dependent
  marking from fields when a subselect is optimized away.