RFR: 8372302: C2: IGVN verification fails because ModXNode::Ideal creates unused intermediate nodes

[Benoît Maillard]

This PR addresses a failure in IGVN verification with `ModI` and `ModL` nodes.

In `ModeXNode::Ideal`, we have code to optimize a modulo expression by expressing it in terms of other operations. There are actually two distinct cases, one where the divisor is a constant and is equal to `modulo 2^k-1` for some integer `k`, and a more general case where other transformations do not succeed. Because these transformations involve creating several new nodes (sometimes in a loop) and calling `phase->transform(...)` on them, we want to avoid accidentally triggering optimizations on the "unfinished" state of the subgraph. For this, we create a temporary dummy node and add edges to the nodes being constructed.

There are some execution paths where the node is not destroyed before `Ideal` returns, and this creates issues during IGVN verification, as the verification code checks if the number of nodes has changed after having called `Ideal` on a given node and not expecting changes.

The path in question is when we exit because the divisor is a constant and is the minimum value:
https://github.com/openjdk/jdk/blob/c19b12927d2ac901ec8ccaa2de5897ee4c47af56/src/hotspot/share/opto/divnode.cpp#L1146-L1147

The zero case does not cause problems (this seems to be because it would hide behind a `div0_check` anyway).

The fix is simply to only create the temporary node when it is needed, and thus avoiding returning without destroying it.

### Testing
- [x] [GitHub Actions](TODO)
- [x] tier1-4, plus some internal testing

Thank you for reviewing!

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Commit messages:
 - Update bug number format
 - Fix style
 - Add reduced test and fix

Changes: https://git.openjdk.org/jdk/pull/28488/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28488&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372302
  Stats: 70 lines in 2 files changed: 63 ins; 4 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/28488.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28488/head:pull/28488

PR: https://git.openjdk.org/jdk/pull/28488