Fix map union optimization for open maps

[gldubc]

An example of the bus is:

closed = closed_map(a: integer(), b: atom())
open = open_map(a: integer(), b: boolean())

union(closed, open)

The optimizer saw that boolean() is a subtype of atom() (and integer() of integer()) and concluded the open map was contained by the closed map. But that is false: the open map also includes maps with extra keys.

The fix is to make the final subtype shortcut reject that tag mismatch, so if the fields suggest “left is subtype of right” but left is :open and right is :closed, return :none. And vice versa. So the generic union is used then.

[josevalim]

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