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

[Anton Seoane Ampudia]

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

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Commit messages:
 - Remove more dead code
 - Remove dead code

Changes: https://git.openjdk.org/jdk/pull/28473/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28473&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8280283
  Stats: 21 lines in 4 files changed: 0 ins; 19 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/28473.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28473/head:pull/28473

PR: https://git.openjdk.org/jdk/pull/28473