Integrated: 8355363: [BACKOUT] 8354668: Missing REX2 prefix accounting in ZGC barriers leads to incorrect encoding

[Tobias Hartmann]

I'm backing out [JDK-8354668](https://bugs.openjdk.org/browse/JDK-8354668) because we're seeing failures with ZGC.

Thanks,
Tobias

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Commit messages:
 - Revert "8354668: Missing REX2 prefix accounting in ZGC barriers leads to incorrect encoding"

Changes: https://git.openjdk.org/jdk/pull/24815/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24815&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8355363
  Stats: 16 lines in 4 files changed: 5 ins; 5 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/24815.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24815/head:pull/24815

PR: https://git.openjdk.org/jdk/pull/24815