[jdk22] RFR: 8324688: C2: Disable ReduceAllocationMerges by default

[Christian Hagedorn]

Due to several recent bug reports after the integration of [JDK-8287061](https://bugs.openjdk.org/browse/JDK-8287061) in JDK 22 (latest one being [JDK-8322854](https://bugs.openjdk.org/browse/JDK-8322854)), we've decided together with @JohnTortugo that we want to minimize the risk and that it's best to disable `ReduceAllocationMerges` by default for JDK 22 which disables the optimizations of [JDK-8287061](https://bugs.openjdk.org/browse/JDK-8287061). 

Thanks,
Christian

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Commit messages:
 - 8324688: C2: Disable ReduceAllocationMerges by default

Changes: https://git.openjdk.org/jdk22/pull/97/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk22&pr=97&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324688
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk22/pull/97.diff
  Fetch: git fetch https://git.openjdk.org/jdk22.git pull/97/head:pull/97

PR: https://git.openjdk.org/jdk22/pull/97