RFR: 8289126: Cleanup unnecessary null comparison before instanceof check in jdk.hotspot.agent

[Andrey Turbanov]

Update code checks both non-null and instance of a class in jdk.hotspot.agent module classes.
The checks and explicit casts could also be replaced with pattern matching for the instanceof operator.

For example, the following code:

              Object node = tree.getLastSelectedPathComponent();
              if (node != null && node instanceof SimpleTreeNode) {
                showInspector((SimpleTreeNode)node);
              }

Can be simplified to:

              Object node = tree.getLastSelectedPathComponent();
              if (node instanceof SimpleTreeNode simpleNode) {
                showInspector(simpleNode);
              }


See similar cleanup in java.base - [JDK-8258422](https://bugs.openjdk.java.net/browse/JDK-8258422)

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Commit messages:
 - 8289126: Cleanup unnecessary null comparison before instanceof check in jdk.hotspot.agent

Changes: https://git.openjdk.org/jdk/pull/9272/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9272&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289126
  Stats: 62 lines in 18 files changed: 0 ins; 13 del; 49 mod
  Patch: https://git.openjdk.org/jdk/pull/9272.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9272/head:pull/9272

PR: https://git.openjdk.org/jdk/pull/9272