next: Let of lists breaks UnrollReduce pass

[tehrengruber]

Expressions like

let _cs_1 = neighbors(V2Eₒ, _inps1)
  reduce(λ(a, b) → a+b, 0)(_cs_1, _cs_1))(_cs_1)
end

currently break the UnrollReduce pass as it is unable to detect the neighbor dimension when only presented with _cs_1. For now we just avoid creating such expression in the CSE. This surfaced in #1793.