RFR: 8294356: IGV: scheduled graphs contain duplicated elements

[Roberto Castañeda Lozano]

This changeset removes duplicated nodes and edges from graph dumps that include a control-flow graph:
![cfg-before-after](https://user-images.githubusercontent.com/8792647/192294554-73ca3927-dab3-4d8f-9503-0904a4da7434.png)
This is achieved by ensuring that HotSpot only visits each node once when dumping IGV graphs.

#### Testing
- Tested that tens of thousands of graphs do not contain duplicated nodes or edges by instrumenting IGV and running `java -Xcomp -XX:PrintIdealGraphLevel=4`.
- Tested manually that unscheduled graphs are not affected by this changeset.
- Tested that running compiler tests with `-XX:PrintIdealGraphLevel=3` does not trigger any failure.

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Commit messages:
 - Do not visit the same node twice

Changes: https://git.openjdk.org/jdk/pull/10423/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10423&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294356
  Stats: 17 lines in 1 file changed: 5 ins; 7 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/10423.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10423/head:pull/10423

PR: https://git.openjdk.org/jdk/pull/10423