RFR: 8309717: C2: Remove Arena::move_contents usage

[Johan Sjölen]

Hi,

Instead of using `Arena::move_contents` we can just see the arena swap as a form of double buffering, reducing this to a pointer swap and a clear. This allows us to remove `Arena::move_contents`, cleaning up the arena code.

Since this requires allocating another pointer for `Compile`, I took the time to move some members around in order to reduce the padding. This means that this patch does *not* introduce a size change for `Compile`.

I'm currently running tier1-3 tests.

Thanks for considering this,
Johan

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Commit messages:
 - Rewrite the comments to reflect current source
 - Move around members to reduce padding
 - Do a pointer swap instead

Changes: https://git.openjdk.org/jdk/pull/14391/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14391&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309717
  Stats: 53 lines in 5 files changed: 19 ins; 22 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/14391.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14391/head:pull/14391

PR: https://git.openjdk.org/jdk/pull/14391