RFR: 8294356: IGV: scheduled graphs contain duplicated elements [v2]

[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.

Roberto Castañeda Lozano has updated the pull request incrementally with one additional commit since the last revision:

  Use nullptr everywhere, remove empty line

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/10423/files
  - new: https://git.openjdk.org/jdk/pull/10423/files/2e80c21f..be726cc9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10423&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10423&range=00-01

  Stats: 4 lines in 1 file changed: 0 ins; 1 del; 3 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