RFR: 8373054: Shenandoah: Remove unnecessary BarrierSetNMethod::arm in shenandoahCodeRoots [v2]

[Ben Taylor]

> The call to arm is redundant, and can be replaced with an assert to ensure the precondition remains true.
> 
> The same set of tier1 tests pass before and after this change with a fastdebug and Shenandoah GC.

Ben Taylor has updated the pull request incrementally with one additional commit since the last revision:

  Fix misleading comment in previous commit

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/28648/files
  - new: https://git.openjdk.org/jdk/pull/28648/files/d830a0a1..a1a9bf11

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28648&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28648&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28648.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28648/head:pull/28648

PR: https://git.openjdk.org/jdk/pull/28648