RFR: 8305755: [JVMCI] missing barriers in CompilerToVM.readFieldValue for Reference.referent [v3]

[Tom Rodriguez]

> Add missing GC barrier for reflective read.  I'm not sure the idiom I've chosen it the correct one so please correct me if there's a better way to write this.  In testing, this resolved the issue.

Tom Rodriguez has updated the pull request incrementally with one additional commit since the last revision:

  Add MO_SEQ_CST for primtive types

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13389/files
  - new: https://git.openjdk.org/jdk/pull/13389/files/3cd12b39..16856123

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13389&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13389&range=01-02

  Stats: 6 lines in 1 file changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/13389.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13389/head:pull/13389

PR: https://git.openjdk.org/jdk/pull/13389