RFR: 8264546: Dependencies: Context class is always an InstanceKlass

[Vladimir Ivanov]

A trivial refactoring/cleanup: dependency context is always an InstanceKlass, but `Dependencies` uses `Klass*`. 
Migrate` Dependencies` from `Klass*` to `InstanceKlass*`.

Testing: hs-tier1 - hs-tier2

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Commit messages:
 - Convert context_type from Klass* to InstanceKlass*

Changes: https://git.openjdk.java.net/jdk/pull/3293/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3293&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264546
  Stats: 33 lines in 4 files changed: 1 ins; 6 del; 26 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3293.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3293/head:pull/3293

PR: https://git.openjdk.java.net/jdk/pull/3293