Issue:
------
VALUES doesn't have a type() function and is considered a
Item_field.

Solution for 5.7:
-----------------
Add a new type() function for Item_values_insert.

On 8.0 and trunk it was fixed by Mithun's Bug#19601973.

Solution for 5.6:
-----------------
Additionally Bug#17458914 is backported.

This will address the problem of using VALUES() in
INSERT ... ON DUPLICATE KEY UPDATE. Create a field object
only if it is in the UPDATE clause, else return a NULL
item.

This will also address the problems mentioned in
Bug#14789787 and Bug#16756402.

Solution for 5.5:
-----------------
As mentioned above Bug#17458914 is backported.

Additionally Bug#14786324 is also backported.

When VALUES() is detected outside its meaningful place,
it should be treated as NULL and is thus replaced with a
Field_null object, with the same name as the original
field.

Fields with type NULL are generally not handled well inside
the server (e.g Innodb will not accept them and it is
impossible to create them in regular tables). So create a
new const NULL item instead.