RFR: 8258625: [JVMCI] refactor and unify JVMCI readFieldValue path

[Tom Rodriguez]

This was originally just a cleanup but it turned out to be a partial safety fix for folding reads during compilation.  I've rolled in a couple other safety checks that were found along the way, along with jtreg test improvements that exercise the new paths.

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Commit messages:
 - 8258625: [JVMCI] refactor and unify JVMCI readFieldValue path

Changes: https://git.openjdk.java.net/jdk/pull/3659/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3659&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258625
  Stats: 799 lines in 17 files changed: 170 ins; 529 del; 100 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3659.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3659/head:pull/3659

PR: https://git.openjdk.java.net/jdk/pull/3659