RFR: 8272723: Don't use Access API to access primitive fields

[Roman Kennke]

For earlier incarnations of Shenandoah, we needed to put barriers before accessing primitive fields. This is no longer necessary nor implemented/used by any GC, and we should simplify the code to do plain access instead.

(We may want to remove remaining primitive access machinery in the Access API soon)

Testing:
 - [ ] tier1
 - [ ] tier2
 - [ ] hotspot_gc

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Commit messages:
 - 8272723: Don't use Access API to access primitive fields

Changes: https://git.openjdk.java.net/jdk/pull/5187/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5187&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272723
  Stats: 24 lines in 2 files changed: 6 ins; 0 del; 18 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5187.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5187/head:pull/5187

PR: https://git.openjdk.java.net/jdk/pull/5187