I've just checked a v10 database and that Index does exist. What I would suggest is dropping the index, performing the upgrade and then re-creating the index - I've copied the create script from a v10 database below.
/****** Object: Index [IX_OM_Contact_ContactPersonaID_ContactLastName] Script Date: 17/08/2017 13:07:56 ******/
CREATE NONCLUSTERED INDEX [IX_OM_Contact_ContactPersonaID_ContactLastName] ON [dbo].[OM_Contact]
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON)