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

[Christian Hagedorn]

On Fri, 26 Sep 2025 09:48:57 GMT, Roberto Castañeda Lozano <rcastanedalo at openjdk.org> wrote:

> 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.

Nice! Looks good to me, too.

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Marked as reviewed by chagedorn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/27520#pullrequestreview-3272344543