Control flow cache fix

[ahejlsberg]

With this PR we only cache control flow type information in loop-back junction labels. Previously we'd cache in all junction labels, but that would sometimes prevent evolving types from being observed. For example:

let x: string | undefined;
while (true) {
    if (cond) {
        x = "";
    }
    else {
        if (x) {
            x.length;
        }
        if (x) {
            x.length;  // Would previously error here
        }
    }
}

Previously we'd error in the code above because we would have cached the type undefined for x in the junction label of following the preceding if statement.

[mhegazy]

👍