RFR: 8369646: Detection of redundant conversion patterns in add_users_of_use_to_worklist is too restrictive

[Benoît Maillard]

This PR addresses a missed optimization opportunity in `PhaseIterGVN`. The missed optimization is the simplification of redundant conversion patterns of the shape `ConvX2Y->ConvY2X->ConvX2Y`.

This optimization pattern is implemented as an ideal optimization on `ConvX2Y` nodes. Because it depends on the input of the input of the node in question, we need to have an appropriate notification mechanism in `PhaseIterGVN::add_users_of_use_to_worklist`. The notification for this pattern was added in [JDK-8359603](https://bugs.openjdk.org/browse/JDK-8359603).

However, that fix was based on the wrong assumption that in `PhaseIterGVN::add_users_of_use_to_worklist`, argument `n` is already the optimized node. However in some cases this argument is actually the node that is about to get replaced.

This happens for example in `PhaseIterGVN::transform_old`. If we find that node `k` returned by `Ideal` actually already exists by calling `hash_find_insert(k)`, we call `add_users_to_worklist(k)`.
As we replace node `k` with `i`, and `i` as a different opcode than `k`, then we cannot use the opcode of `k` to detect the redundant conversion pattern. 

```c++
...
  // Global Value Numbering
  i = hash_find_insert(k);      // Check for pre-existing node
  if (i && (i != k)) {
    // Return the pre-existing node if it isn't dead
    NOT_PRODUCT(set_progress();)
    add_users_to_worklist(k);
    subsume_node(k, i);       // Everybody using k now uses i
    return i;
  }
...


The bug was quite intermittent and only showed up in some cases with `-XX:+StressIGVN`.

### Proposed Fix

We make the detection of the pattern less specific by only looking at the opcode of the user of `n`, and not directly the opcode of `n`. This is consistent with the detection of other patterns in `PhaseIterGVN::add_users_of_use_to_worklist`.

### Testing
- [x] [GitHub Actions](https://github.com/benoitmaillard/jdk/actions?query=branch%3AJDK-8369646)
- [x] tier1-3, plus some internal testing
- [x] Added a second run for the existing test with `-XX:+StressIGVN` and a fixed stress seed

Thank you for reviewing!

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

Commit messages:
 - 8369646: Fix excissively restrictive pattern in add_users_of_use_to_worklist
 - 8369646: Add second test run with -XX:+StressIGVN and stress seed

Changes: https://git.openjdk.org/jdk/pull/27900/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27900&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369646
  Stats: 15 lines in 2 files changed: 9 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/27900.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27900/head:pull/27900

PR: https://git.openjdk.org/jdk/pull/27900