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

[Srinivas Vamsi Parasa]

> 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

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/26009/files
  - new: https://git.openjdk.org/jdk/pull/26009/files/2dba6e3f..de7c373f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26009&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26009&range=00-01

  Stats: 70 lines in 1 file changed: 0 ins; 19 del; 51 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