RFR: 8368753: IGV: improve CFG view of difference graphs [v2]

[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.

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

  Update copyright header

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27520/files
  - new: https://git.openjdk.org/jdk/pull/27520/files/a35f357b..8cdff704

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27520&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27520&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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