Another case of missing alias tracking

[athas]

let main n =
  let (a,b) = let y = iota n
              in (y,y)
  let a[0] = 0
  let b[0] = 0
  in (a,b)

Interestingly, the type checker complains as expected if we try to return (a,b) as two unique arrays, so we do actually track that a and b are aliased.