[jdk11u-dev] RFR: 8232782: Shenandoah: streamline post-LRB CAS barrier (aarch64)

[Aleksey Shipilev]

Unclean backport to improve Shenandoah AArch64 performance and provide grounds for subsequent backports. 

There were conflicts in `shenandoah_aarch64.ad`, as there is `weakCompareAndSwapPAcq_shenandoah` and `weakCompareAndSwapNAcq_shenandoah` are missing in 11u.

Additional testing:
 - [x] Linux AArch64 `hotspot_gc_shenandoah` 
 - [x] Linux AArch64 `tier1` with `-XX:+UseShenandoahGC`
 - [x] Linux AArch64 `tier2` with `-XX:+UseShenandoahGC`
 - [ ] Linux AArch64 jcstress `-m quick` with `-XX:+UseShenandoahGC`

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Commit messages:
 - Backport c5bb02358472a993fbefe816302411483273656c

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/268/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=268&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8232782
  Stats: 184 lines in 4 files changed: 139 ins; 6 del; 39 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/268.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/268/head:pull/268

PR: https://git.openjdk.java.net/jdk11u-dev/pull/268