Ensure no compiler visit method tries to access .statement

Ensure no compiler visit method tries to access .statement

Fixed structural compiler issue where some constructs such as MySQL /
PostgreSQL "on conflict / on duplicate key" would rely upon the state of
the :class:`_sql.Compiler` object being fixed against their statement as
the top level statement, which would fail in cases where those statements
are branched from a different context, such as a DDL construct linked to a
SQL statement.

Fixes: #5656
Change-Id: I568bf40adc7edcf72ea6c7fd6eb9d07790de189e