RFR: 8271579: G1: Move copy before CAS in do_copy_to_survivor_space

[Hamlin Li]

Propose to move copy before CAS in do_copy_to_survivor_space, as we found this will improve G1 performance. Specjbb shows 3.7% in critical on aarch64, no change in max.

After this change copy_to_survivor in G1 is also aligned with PS's copy_to_survivor.

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Commit messages:
 - G1: move copy before CAS to improve performance

Changes: https://git.openjdk.java.net/jdk/pull/4983/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4983&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271579
  Stats: 2 lines in 1 file changed: 1 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4983.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4983/head:pull/4983

PR: https://git.openjdk.java.net/jdk/pull/4983