RFR: 8336944: Shenandoah: Should only relativize stack chunks for successful evacuations [v2]

[William Kemper]

> In some cases, different threads may race to evacuate an object. The race is won by "CAS"ing in the forwarding pointer. The threads that lose this race must "back out" their allocation. The work to relativize stack chunks should only happen for the thread that wins the evacuation race.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Assert that return value is correct after relativization of stack chunk

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20288/files
  - new: https://git.openjdk.org/jdk/pull/20288/files/ad832e7e..6943b79a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20288&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20288&range=00-01

  Stats: 2 lines in 1 file changed: 1 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/20288.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20288/head:pull/20288

PR: https://git.openjdk.org/jdk/pull/20288