Catch wrong use of ==/!= and equals()/!equals() in equals() implementations

[leventov]

Bugs like this: #8326 could be caught on the static analysis level, using Regex patterns (Checkstyle Regexp) or Structural search inspection.

Example of regex which should have caught #8326: (?m)return ([^\;]+)(this\.)?([a-zA-Z]+) \!\= [a-zA-Z]+\.\1

Similar regexes could be created for equals(), Object.equals(), as well as incorrect use in

if (x != that.x) {
  return false;
}

code in equals() implementations.