Control flow narrowing sometimes de-aliases unions

[sandersn]

After #42149, which usually preserves the type aliases of unions as written, some type aliases end up de-aliased in control flow:
(example based on @types/amap-js-api)

type Direction = 'up' | 'down' | 'left' | 'right'
interface Input {
    direction?: Direction
}
declare let input: Input
if (input.direction) {
    input.direction/*should be Direction*/
}

Expected: The inner input.direction: Direction

Actual: The inner input.direction: 'up' | 'down' | 'left' | 'right'

This occurs because control flow narrowing uses filterType, which doesn't understand type aliases or attempt to preserve them. Previously the union-interning behaviour preserved the alias, but that's no longer the case.

[sandersn]

This is a regression in 4.2, although after discussing with @ahejlsberg, we agree that it's one worth taking if there's no obvious way to fix it.

[sandersn]

@ahejlsberg, I'm assigning you for the 4.2 milestone so that @RyanCavanaugh and @DanielRosenwasser are aware of the regression, but feel free to close this when you've had a chance to decide whether it's worth fixing.