RFR: 8274232: Cleanup unnecessary null comparison before instanceof check in jdk.jdi

[Andrey Turbanov]

Update code checks both non-null and instance of a class in jdk.jdi module classes.
The checks and explicit casts could also be replaced with pattern matching for the instanceof operator. 
Similar cleanups
1. [JDK-8273484](https://bugs.openjdk.java.net/browse/JDK-8273484) java.naming
2. [JDK-8258422](https://bugs.openjdk.java.net/browse/JDK-8258422) java.base

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Commit messages:
 - 8274232: Cleanup unnecessary null comparison before instanceof check in jdk.jdi

Changes: https://git.openjdk.java.net/jdk/pull/5659/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5659&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274232
  Stats: 65 lines in 20 files changed: 0 ins; 13 del; 52 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5659.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5659/head:pull/5659

PR: https://git.openjdk.java.net/jdk/pull/5659