RFR: 8280283: Dead compiler code found during the JDK-8272058 code review [v2]

[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

Anton Seoane Ampudia has updated the pull request incrementally with one additional commit since the last revision:

  Delete more unused code

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/28473/files
  - new: https://git.openjdk.org/jdk/pull/28473/files/749eda78..696cdd01

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28473&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28473&range=00-01

  Stats: 10 lines in 2 files changed: 0 ins; 7 del; 3 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