8371881: C2: Fix potential SEGV in VTransformReductionVectorNode tracing

[vish-chan]

This PR fixes a potential SEGV and removes dead code:
• Fix: Prevent potential SEGV in VTransformReductionVectorNode - [from @shipilev] This fixes a crash in diagnostic code when isa_ReductionVector() unexpectedly returns nullptr. While this indicates the graph is already corrupted, the additional crash in TRACE_OPTIMIZE makes debugging harder. The fix adds defensive null checking to prevent the diagnostic crash and improve error handling.

• Cleanup: Remove unused first_red variable

---

Progress

• Change must not contain extraneous whitespace
• Commit message must refer to an issue
• Change must be properly reviewed (2 reviews required, with at least 1 Reviewer, 1 Author)

Issue

• JDK-8371881: C2: Fix potential SEGV in VTransformReductionVectorNode tracing (Enhancement - P5)

Reviewers

• Aleksey Shipilev (@shipilev - Reviewer)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/28323/head:pull/28323
$ git checkout pull/28323

Update a local copy of the PR:
$ git checkout pull/28323
$ git pull https://git.openjdk.org/jdk.git pull/28323/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 28323

View PR using the GUI difftool:
$ git pr show -t 28323

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/28323.diff

Using Webrev

Link to Webrev Comment

[bridgekeeper]

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[openjdk]

@vish-chan This change is no longer ready for integration - check the PR body for details.

[openjdk]

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[mlbridge]

Webrevs

• 01: Full - Incremental (0824593d)
• 00: Full (f6947173)

[shipilev]

Looks fine, but @eme64 should take a look as well.

/reviewers 2

[openjdk]

@shipilev
The total number of required reviews for this PR (including the jcheck configuration and the last /reviewers command) is now set to 2 (with at least 1 Reviewer, 1 Author).

[eme64]

@vish-chan @shipilev thanks for working on this!

Looks reasonable to me :)

[vish-chan]

@eme64 I'm not sure but your approval might be required.