[Flexible constructors] Incorrect diagnostic: The final field x may already have been assigned

I am not claiming snippets like below are _realistic_, but it does reveal _real_ problem in flow analysis for flexible constructors:

The following program is rejected by 4.40, 4.41 M2 and master/HEAD with 2 errors of the form `The final field x may already have been assigned` while javac compiles them correctly. See that there are no other assignments to `x` and `z` in the program, so these messages are plain wrong:

```
public final class X {
final int x; // 1
final int y; // 2
final int z; // 4

{
x = 10;
z = y;
}

X() {
int xx = 123; // 8
if (true)
throw new RuntimeException();
super();
}
X(int a) {
this.y = 10;
if (true)
throw new RuntimeException();
super();
}
}

```