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_select.cc

@@ -2413,6 +2413,12 @@ JOIN::optimize_inner()
           thd, &Item::varchar_upper_cmp_transformer);
   }
 
+  if (substitute_indexed_vcols_for_join(this))
+  {
+    error= 1;
+    DBUG_RETURN(1);
+  }
+
   conds= optimize_cond(this, conds, join_list, ignore_on_expr,
                        &cond_value, &cond_equal, OPT_LINK_EQUAL_FIELDS);