Make sure we don't remove conjuncts that discriminate between
disjunction branches.

issue 2544

-- a.cue --
#A: value?: {x: number, y: number} | *{x: 0, y: 0} | 5
a: #A & {value: {}}
-- out/trim --
== a.cue
#A: value?: {x: number, y: number} | *{x: 0, y: 0} | 5
a: #A & {value: {}}
