RFR: 8280283: Dead compiler code found during the JDK-8272058 code review

[Anton Seoane Ampudia]

On Mon, 24 Nov 2025 09:26:13 GMT, Anton Seoane Ampudia <aseoane at openjdk.org> wrote:

> This PR removes some dead code that was found during review for [JDK-8272058](https://bugs.openjdk.org/browse/JDK-8272058).
> 
> `target_addr_for_insn_or_null` is never run with a `ldrw` to `zr` (i.e. a safepoint poll). This is just a remnant from global safepointing, before we moved to using thread-local handshakes. No safepoint polling code reaches this function. More information can be read in the [original code review](https://github.com/openjdk/jdk18/pull/51#discussion_r774922087). Additionally, I have run tiers 1-6 to make sure this path did not exercise.
> 
> This changeset also cleans up the unused `is_nop` function, following the comments in the issue. Other dead code mentioned there has since been long disappered.
> 
> **Testing:** passes tiers 1-4

Pinging @bulasevich @theRealAph, as they were in the original code review and discussed about this.

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PR Comment: https://git.openjdk.org/jdk/pull/28473#issuecomment-3582016397