RFR: 8361144: Strenghten the Ideal Verification in PhaseIterGVN::verify_Ideal_for by comparing the hash of a node before and after Ideal [v2]

[Benoît Maillard]

> This PR adds a node hash comparison after calling `Ideal` in `PhaseIterGVN::verify_Ideal_for` to introduce an additional layer of verification for missed optimizations. Previously, we relied on the return value of `Ideal`, which is expected to be `nullptr` if no transformation was done.
> 
> By also checking the node's hash before and after `Ideal`, we could catch inconsistencies in the implementation or unintended modifications to the graph. Both of these may indicate missed or incomplete optimizations.
> 
> ### Testing
> - [x] [GitHub Actions](https://github.com/benoitmaillard/jdk/actions?query=branch%3AJDK-8361144)
> - [x] tier1-3, plus some internal testing
> 
> Thank you for reviewing!

Benoît Maillard has updated the pull request incrementally with one additional commit since the last revision:

  8361144: update comment
  
  Co-authored-by: Damon Fenacci <damon.fenacci at oracle.com>

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/26064/files
  - new: https://git.openjdk.org/jdk/pull/26064/files/e06b4d53..28851936

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26064&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26064&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/26064.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26064/head:pull/26064

PR: https://git.openjdk.org/jdk/pull/26064