RFR: 8368303: AlwaysAtomicAccesses is excessively strict

[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.

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Commit messages:
 - First try

Changes: https://git.openjdk.org/jdk/pull/27432/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27432&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368303
  Stats: 11 lines in 1 file changed: 9 ins; 0 del; 2 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