CFA should consider branch flags

This is a suggestion to improve the CFA. It would be good if TypeScript considered branch flags when analysing code paths.

function f() {
    let text: string;
    let hasTakenVerySpecificBranch = false;
    if (0.5 < Math.random()) {
        if (0.5 < Math.random()) {
            if (0.5 < Math.random()) {
                hasTakenVerySpecificBranch = true;
                text = '';
            }
        }
    }
    if (!hasTakenVerySpecificBranch) {
        text = ''
    }
    console.log(text); // Error 'text' is used before assigned.
}

We assign text on all possible code paths above. And it shouldn't error above.

A branch flag is simply a boolean variable that is considered when doing the control flow analysis.

Start by reproducing the TypeScript example and confirming the current definite-assignment error. Read the compiler's control-flow analysis implementation and existing tests for definite assignment and boolean narrowing. Done means branch-flag conditions are incorporated without regressing related control-flow behavior, with tests covering the example.