RFR: 8354300: Fields in String are not trusted

[Per Minborg]

On Mon, 14 Apr 2025 14:47:59 GMT, Per Minborg <pminborg at openjdk.org> wrote:

> This PR proposes to add the `@Stable` annotation to `j.l.String.hash` and `j.l.String.hashIsZero`. This means the VM can trust these fields to never change which enables constant folding optimizations.
> 
> This PR is tested in tier1, tier2, tier3, and tier4 which all pass.

With the proposed changes, the JVM is able to constant-fold the hash value. See:

https://github.com/openjdk/jdk/pull/23972#issuecomment-2793199126

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PR Comment: https://git.openjdk.org/jdk/pull/24625#issuecomment-2802361777