RFR: 8368753: IGV: improve CFG view of difference graphs

[Roberto Castañeda Lozano]

This changeset improves the control-flow graph view of difference graphs by:

1. ensuring that nodes are scheduled locally within each block, and
2. hiding internal, artificial blocks containing nodes that remain in the graph even if they are dead, such as the top constant node.

The following screenshot illustrates the effect of scheduling nodes locally:

<img width="3853" height="1033" alt="JDK-8368753" src="https://github.com/user-attachments/assets/bdc0f6de-3d28-4615-9e0d-221de2ad4770" />

For example, before this changeset (left) the `Return` node in B9 is scheduled at the beginning of the block. After the changeset (right), this node is scheduled last, as expected.

**Testing:** tier1 and manual testing on a few graphs.

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Commit messages:
 - Schedule difference graphs locally and mark their artificial block

Changes: https://git.openjdk.org/jdk/pull/27520/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27520&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368753
  Stats: 64 lines in 4 files changed: 44 ins; 11 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/27520.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27520/head:pull/27520

PR: https://git.openjdk.org/jdk/pull/27520