RFR: 8360775: Fix Shenandoah GC test failures when APX is enabled [v2]

[Emanuel Peter]

On Mon, 30 Jun 2025 22:19:18 GMT, Srinivas Vamsi Parasa <sparasa at openjdk.org> wrote:

>> 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 does not restore the contents of rax.
>> 
>> Both the issues are fixed in this PR.
>
> Srinivas Vamsi Parasa has updated the pull request incrementally with one additional commit since the last revision:
> 
>   restore the orginal contents of rax

Our internal testing passes. Fix looks reasonable. Thanks for the work!

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Marked as reviewed by epeter (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/26009#pullrequestreview-2985721144