Fix union/union or intersection/intersection type inference

[ahejlsberg]

The reduction step introduced in #5738 is a bit too aggressive. For example:

interface Foo<T> {
    x: T;
}

function bar<T>(x: Foo<T> | string): T {
    return bar(x);
}

In the bar(x) call we infer from Foo<T> | string to Foo<T> | string which previously was reduced to empty types (and thus no inferences) because all constituents are pairwise identical. Therefore, the above would be an error. With this PR we instead find all of the pairwise identical constituents and infer from each match to itself. We then remove the pairwise matches and proceed with the already existing inference algorithm.

[mhegazy]

👍

[vladima]

👍