RFR: 8264548: Dependencies: ClassHierarchyWalker::is_witness() cleanups

[Vladimir Ivanov]

Simplify `ClassHierarchyWalker::is_witness()`.

Testing: hs-tier1 - hs-tier4.

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Commit messages:
 - Refactor ClassHierarchyWalker::is_witness()
 - Convert context_type from Klass* to InstanceKlass*

Changes: https://git.openjdk.java.net/jdk/pull/3297/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3297&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264548
  Stats: 68 lines in 4 files changed: 17 ins; 12 del; 39 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3297.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3297/head:pull/3297

PR: https://git.openjdk.java.net/jdk/pull/3297