RFR: 8309717: C2: Remove Arena::move_contents usage [v2]

[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

Johan Sjölen has updated the pull request incrementally with two additional commits since the last revision:

 - Update src/hotspot/share/opto/matcher.cpp
   
   Co-authored-by: Tobias Hartmann <tobias.hartmann at oracle.com>
 - Update src/hotspot/share/opto/compile.hpp
   
   Co-authored-by: Tobias Hartmann <tobias.hartmann at oracle.com>

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/14391/files
  - new: https://git.openjdk.org/jdk/pull/14391/files/565b3df4..a2f6aee4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14391&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14391&range=00-01

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 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