SQL Server Failover Cluster Instances are not mandatory for AGs. All you need is a Windows Cluster and standalone SQL Instances to make it work(All the Servers(nodes) participating in AlwaysON AG should be part of same Windows cluster). You can build AGs on top of SQL clustered Instances as well(same as we used to do with DB Mirroring), it really depends on your business RTOs/RPOs. Consider AG as "Super Mirroring". They both work pretty much on same principals, but AG has some Super powers like "Multiple databases Failover as a single unit", "readable Secondaries" etc...

Bottom Line: SQL Server failover Clustered Instances are not required for setting up AGs.