RFR: 8368303: AlwaysAtomicAccesses is excessively strict [v3]

[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 two additional commits since the last revision:

 - Review feeback
 - Review feeback

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27432/files
  - new: https://git.openjdk.org/jdk/pull/27432/files/f61f6fce..7399fc59

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27432&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27432&range=01-02

  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 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