RFR: 8368303: AlwaysAtomicAccesses is excessively strict [v2]

[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:

  Next try

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/27432/files
  - new: https://git.openjdk.org/jdk/pull/27432/files/1e891b76..f61f6fce

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27432&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27432&range=00-01

  Stats: 13 lines in 1 file changed: 5 ins; 4 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