RFR: 8325672: C2: allocate PhaseIdealLoop::_loop_or_ctrl from C->comp_arena()

[Emanuel Peter]

If we don't do that, then this array is ResourceArea allocated. That means we can get in trouble with ResourceMarks inside the PhaseIdealLoop. If the array is updated because of new nodes, and this grows the area in a ResourceMark scope, then the data behind _loop_or_ctrl becomes invalid, and we get use-after-free memory corruption bugs.

The array was added in [JDK-8302670](https://bugs.openjdk.org/browse/JDK-8302670), so we should backport down to JDK21.

I don't yet have a reproducer. But this triggered with my patch for [JDK-8325589](https://bugs.openjdk.org/browse/JDK-8325589). See https://github.com/openjdk/jdk/pull/17800#discussion_r1486534950.

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

Commit messages:
 - 8325672

Changes: https://git.openjdk.org/jdk/pull/17814/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17814&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325672
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17814.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17814/head:pull/17814

PR: https://git.openjdk.org/jdk/pull/17814