v5: A second polymorphism optimization opportunity

ref: `grafast/grafast/__tests__/dcc/queries/item-lootbox-minimum.test.graphql`

Review the plan diagram and experiment with a new way of handling the polymorphism here.

Lootbox is not polymorphic but Item is.

Currently the plan has more parallel paths than it should as many of the paths could theoretically be collapsed down or deduplicated such that only one path is executed, but not using the current way of deduplication due to the way the schema is written.