Compare and contrast with 3 and 30.

The conjuncts in x and y are directly reachable (i.e. without going
via z) from the evaluation result. So they can't be removed. Because x
survives, we also keep all uses of x. So z: x cannot be removed.

-- a.cue --
x: string
y: "hi"
z: x
z: y
-- out/trim --
== a.cue
x: string
y: "hi"
z: x
z: y
