consider indpendent CTE for UPDATE..FROM

consider indpendent CTE for UPDATE..FROM

Fixed issue where referring to a FROM entry in the SET clause of an UPDATE
statement would not include it in the FROM clause of the UPDATE statement,
if that entry were nowhere else in the statement; this occurs currently for
CTEs that were added using :meth:`.Update.add_cte` to provide the desired
CTE at the top of the statement.

Fixes: #10408
Change-Id: I6e3c6ca7a00cc884bda7e0f24c62c34c75134e5b