RFR: 8338881: GenShen: Use explicit third temp register for post barrier

[William Kemper]

Conflict with IU barrier support in 21.

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Commit messages:
 - Backport 12af9f629ab2f3d29392eb8e6ab2abd518723480

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/97/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=97&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338881
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/97.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/97/head:pull/97

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/97