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

[Christian Hagedorn]

On Mon, 29 Sep 2025 08:47:25 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.
>
> Roberto Castañeda Lozano has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update copyright header

Marked as reviewed by chagedorn (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/27520#pullrequestreview-3303286849