RFR: 8360775: Fix Shenandoah GC test failures when APX is enabled

Fri Jun 27 16:34:11 UTC 2025

This PR fixes the test failures seen in many JTreg tests related to Shenandoah GC (`test/hotspot/jtreg/gc/shenandoah/`) with UseAPX. The issues were root caused to: 

1. Higher band registers are not saved and restored in Shenandoah load_reference_barrier. 
2. Pusha/Popa implementation using push2p/pop2p 

Both the issues are fixed in this PR.

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Commit messages:
 - 8360775: Fix Shenandoah GC test failures when APX is enabled

Changes: https://git.openjdk.org/jdk/pull/26009/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26009&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8360775
  Stats: 109 lines in 2 files changed: 59 ins; 0 del; 50 mod
  Patch: https://git.openjdk.org/jdk/pull/26009.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26009/head:pull/26009

PR: https://git.openjdk.org/jdk/pull/26009