RFR: 8266973: Migrate to ClassHierarchyIterator when enumerating subclasses

[Vladimir Ivanov]

Replace ad-hoc recursion when enumerating subclasses with `ClassHierarchyIterator`.

Found 3 occurrences:
- `Dependencies::find_finalizable_subclass()`
- `reinitialize_vtable_of()`
- `VM_RedefineClasses::increment_class_counter()`

Testing:
- [x] hs-tier1 - hs-tier4

-------------

Commit messages:
 - 8266973: Migrate to ClassHierarchyIterator when enumerating subclasses

Changes: https://git.openjdk.java.net/jdk/pull/3995/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3995&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266973
  Stats: 55 lines in 7 files changed: 7 ins; 19 del; 29 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3995.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3995/head:pull/3995

PR: https://git.openjdk.java.net/jdk/pull/3995