RFR: JDK-8319256: Print more diagnostic information when an unexpected user is found in a Phi

Thu Nov 2 22:21:14 UTC 2023

Can I please get some review on this PR to:

- Add more verification around the implementation of reduce allocation merges.
- Print more diagnostic information when an unexpected situation is encountered.

Testing: Linux x64 tier1-4, Renaissance, Dacapo and SpecJBB.

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Commit messages:
 - Some small refactor & add VerifyReduceAllocationMerges

Changes: https://git.openjdk.org/jdk/pull/16465/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16465&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319256
  Stats: 100 lines in 3 files changed: 56 ins; 6 del; 38 mod
  Patch: https://git.openjdk.org/jdk/pull/16465.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16465/head:pull/16465

PR: https://git.openjdk.org/jdk/pull/16465