Expressions that match an indexed generated column may be replaced
with the generated column in JOIN::optimize(), which enables use of
the associated index. Single-table update and delete statements don't
have a JOIN data structure, so this optimization is not performed for
such statements.

This patch changes JOIN::substitute_gc() into a free function, which
is used from single-table update and delete statements as well as from
JOIN::optimize(). With this change, single-table update and delete
statements can use indexes on generated columns for filtering and
ordering, even if the statements use the generated column expression
instead of an explicit reference to the column.