RFR: 8255991: Shenandoah: Apply 'weak' LRB on cmpxchg and xchg

[Roman Kennke]

It is possible to access a Reference's referent by using various cmpxchg and xchg intrinsics. When that happens, we need to apply the weak LRB to prevent resurrection.

Testing: hotspot_gc_shenandoah

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Commit messages:
 - 8255991: Shenandoah: Apply 'weak' LRB on cmpxchg and xchg

Changes: https://git.openjdk.java.net/jdk/pull/1098/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1098&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255991
  Stats: 8 lines in 3 files changed: 3 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1098.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1098/head:pull/1098

PR: https://git.openjdk.java.net/jdk/pull/1098