when we do NormalizeToSlot, we pushed complex expression and only remain
slot of it. When we do this, we collect alias and their child and
compute its child in bottom project, remain the result slot in current
node. for example
Window(max(...), c1 as a1)
after normalization, we get
Window(max(...), a1)
+-- Project(..., c1 as a1)
But, in some cases, we remove some SlotReference by mistake, for example
Window(max(...), c1, c1 as a1)
after normalization, we get
Window(max(...), a1)
+-- Project(..., c1 as a1)
we lost the SlotReference c1. This PR fix this problem. After this Pr,
we get
Window(max(...), c1, a1)
+-- Project(..., c1, c1 as a1)
b50c1448df