MDEV-35616: Add basic optimizer support for virtual column

(Review input addressed)

After this patch, the optimizer can handle virtual column expressions
in WHERE/ON clauses. If the table has an indexed virtual column:

  ALTER TABLE t1
    ADD COLUMN vcol INT AS (col1+1),
    ADD INDEX idx1(vcol);

and the query uses the exact virtual column expression:

  SELECT * FROM t1 WHERE col1+1 <= 100

then the optimizer will be able use index idx1 for it.

This is achieved by walking the WHERE/ON clauses and replacing instances
of virtual column expression (like "col1+1" above) with virtual column's
Item_field (like "vcol"). The latter can be processed by the optimizer.

Replacement is considered (and done) only in items that are potentially
usable to the range optimizer.

sql/sql_update.cc

@@ -470,6 +470,9 @@ bool Sql_cmd_update::update_single_table(THD *thd)
                                           (uchar *) 0);
   }
 
+  if (conds && substitute_indexed_vcols_for_table(table, conds))
+    DBUG_RETURN(1); // Fatal error
+
   // Don't count on usage of 'only index' when calculating which key to use
   table->covering_keys.clear_all();
   transactional_table= table->file->has_transactions_and_rollback();