RFR: 8368303: AlwaysAtomicAccesses is excessively strict [v4]

[Andrew Haley]

> In C1, the experimental flag `AlwaysAtomicAccesses` treats accesses to all fields as if they were volatile fields. This is correct but excessive: we only need single-copy atomicity to satisfy `AlwaysAtomicAccesses`. 
> 
> We need this in order to allow progress on JDK-8365147.

Andrew Haley has updated the pull request incrementally with one additional commit since the last revision:

  Update src/hotspot/share/gc/shared/c1/barrierSetC1.cpp
  
  Co-authored-by: Dean Long <17332032+dean-long at users.noreply.github.com>

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27432/files
  - new: https://git.openjdk.org/jdk/pull/27432/files/7399fc59..e4cdd74b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27432&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27432&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/27432.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27432/head:pull/27432

PR: https://git.openjdk.org/jdk/pull/27432