Cleanup unnecessary null comparison before instanceof check in java.base

Update code checks both non-null and instance of a class in java.base module classes.
The checks and explicit casts could also be replaced with pattern matching for the instanceof operator.
For example:
    The following code:
        if ((obj != null) && (obj instanceof File)) {
            return compareTo((File)obj) == 0;
    Can be simplified to:
        if (obj instanceof File file) {
            return compareTo(file) == 0;

The initial patch was proposed by Andrey Turbanov(turbanoff):
https://github.com/openjdk/jdk/pull/20