Non-static constant values should be tracked by the sigtest

The signature test can check constant values of the static final fields ("ST" field in the example below) but doesn't check the constant values of the non-static fields ("NST" field in the example below). Since the non-static constant values may effect the source code compatibility (see switch-case example below caused the "duplicate case label" compilation error), they should be tracked by the sigtest as well.

====
class Test {
    public static final int ST = 1;
    public final int NST = 2;
   
}

class Test2 extends Test {
   {
        switch (0) {
            case NST:
            case 2:
        }
    }
}
> duplicate case label
> case 2:
> 1 error