RFR: JDK-8302594: use-after-free in Node::destruct [v2]

[Justin King]

> Instrumenting Arena for ASan revealed some `use-after-free` behavior in C2. One of them is in `Node::destruct`, where the storage for `Node` is free'd and then fields are accessed. Thankfully none of the methods called allocate, but they could in the future. To resolve this, we move the calls to `Afree` to the end of `Node::destruct`.

Justin King has updated the pull request incrementally with one additional commit since the last revision:

  Update based on review
  
  Signed-off-by: Justin King <jcking at google.com>

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12577/files
  - new: https://git.openjdk.org/jdk/pull/12577/files/440ebb30..376feb59

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12577&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12577&range=00-01

  Stats: 17 lines in 1 file changed: 9 ins; 8 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12577.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12577/head:pull/12577

PR: https://git.openjdk.org/jdk/pull/12577