G1: "yielded" is not initialized on some paths after JDK-8140326

SonarCloud has a new warning after JDK-8140326:

```
        bool yielded;
        bool mark_aborted = yield_if_necessary(yielded);
        if (mark_aborted) {
...
        } else if (yielded && !should_rebuild_or_scrub(hr)) { // <--- Branch condition evaluates to a garbage value
...
        }
```

This warning is about `yielded` being not initialized on some paths, reading the garbage value from it on branch checks.

There are other uses of `yield_if_necessary`, and at some uses we initialize `yielded = true`. This seems a bit awkward in itself: we are assuming the yield happened even without checking it in yield_if_necessary. It is therefore unclear what should be the default value in other cases.