RFR: 8345580: Remove const from Node::_idx which is modified

[Yagmur Eren]

`Node::_idx` is declared as `const`, however, it is modified by `Node::set_idx`. Please see: https://github.com/openjdk/jdk/blob/166c12771d9d8c466e73a9490c4eb1fc9a5f6c24/src/hotspot/share/opto/node.hpp#L588

As already stated in [JDK-8345580](https://bugs.openjdk.org/browse/JDK-8345580) issue, this behavior is counterintuitive because a const variable is expected to remain unmodified throughout its lifetime. Additionally, C++ International Standard states that "_...any attempt to modify a `const` object during its lifetime results in undefined behavior._ ". 
To address this, `const` should be removed from the declaration of `Node::_idx` to align with its intended use and avoid violating the C++ standard. Tested with tier1,2,3,4,5.

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Commit messages:
 - 8345580: Remove const from Node::_idx which is modified

Changes: https://git.openjdk.org/jdk/pull/22646/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22646&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345580
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/22646.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22646/head:pull/22646

PR: https://git.openjdk.org/jdk/pull/22646