Issue:
------
When an UPDATE statement is using update on the fly
optimization, generated column related checks are skipped.

Solution:
---------

On 5.7:
Call validate_gc_assignment at an earlier place in
Query_result_update::initialize_tables.

On 8.0 and trunk:
Move the call to validate_gc_assignment
Sql_cmd_update::prepare_inner(). This way no separate
handling is required for single and multi-table UPDATE.