RFR: 8335390: C2 MergeStores: wrong result with Unsafe

[Emanuel Peter]

In JDK23, I introduced `MergeStores` [JDK-8318446](https://bugs.openjdk.org/browse/JDK-8318446) https://github.com/openjdk/jdk/pull/16245.

I now realized that the pointer-parsing has some issues with `Unsafe` stores. Thus, I disable the optimization for those for now.

**Future Work**
In [JDK-8335392](https://bugs.openjdk.org/browse/JDK-8335392), I plan to redesign the pointer-parsing, and enable more features, including `Unsafe` stores.

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Commit messages:
 - need larger heap
 - improve comment
 - 8335390

Changes: https://git.openjdk.org/jdk/pull/19964/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19964&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335390
  Stats: 140 lines in 3 files changed: 137 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/19964.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19964/head:pull/19964

PR: https://git.openjdk.org/jdk/pull/19964