v5: Unneeded branching while using query variables

[jemgillam]

Ref grafast/grafast/__tests__/dcc/queries/npc-friends.test.graphql

These two query plans are for the same query, but in the lower one we are using variables and unneeded branching is occurring.

query NpcFriends {
  npc(id: 301) {
    __typename
    id
    name

    exCrawler
    ... on Manager {
      friends(first: 2) {
        id
        name
      }
    }
    ... on Staff {
      friends(first: 2) {
        id
        name
      }
    }
  }
}

But here, unneeded branching and combining is occurring after a lambda step:

query NpcFriendsWithVariables($first: Int) @variables(values: { first: 2 }) {
  npc(id: 301) {
    __typename
    id
    name

    exCrawler
    ... on Manager {
      friends(first: $first) {
        id
        name
      }
    }
    ... on Staff {
      friends(first: $first) {
        id
        name
      }
    }
  }
}