There's also no need to make it left to right usage, as long as it's acyclic:

  select y-2 as x, 3 as y, y/x as z;

Would this be compiled into a graph of subqueries and window statements?

I'm not following, the original could be written as

  select x + 2 as y, 1 as x, y/x as z;

with the same column values in a different order. Order of arguments shouldn't matter is all I was saying.