RFR: 8282076: Merge some debug agent changes from the loom repo

[Chris Plummer]

There are some minor debug agent changes in the loom repo that are not virtual thread specific, and I would like to merge them into the jdk repo. This will make the future real merge of loom into jdk a bit cleaner. 

The biggest change is moving some code into the new filterAndHandleEvent() function. However, there are no logic changes. It's the same code, just moved into its own function.

In handleMethodEnterEvent() I reworked some confusing logic and also the comment that goes with it, but it should be functionally the same.

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Commit messages:
 - Merge some loom changes that are not vthread specific.

Changes: https://git.openjdk.java.net/jdk/pull/7524/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7524&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282076
  Stats: 116 lines in 3 files changed: 61 ins; 45 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7524.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7524/head:pull/7524

PR: https://git.openjdk.java.net/jdk/pull/7524