Dataflow facts from ExpressionStatements

Follow-up on https://github.com/typetools/checker-framework/issues/3267#issuecomment-619656440

I can reproduce your comment about your new typestate checker using the Regex Checker:

    void bar(String s) {
        RegexUtil.isRegex(s);
        if (true) {
            // :: error: (argument.type.incompatible)
            Pattern.compile(s); // Error missing
        }
    }

I'll look into wrapping all ExpressionStatements into an assignment to a temporary variable, turning the above code into:

    void bar(String s) {
        boolean res = RegexUtil.isRegex(s);
        if (true) {
            // :: error: (argument.type.incompatible)
            Pattern.compile(s);
        }
    }

which correctly gives an error.