RFR: Refactor IRIW tests a bit

[Aleksey Shipilev]

This refactors the IRIW tests: makes sure the operands are in order, names unsyncronized test correct, introduces opaque test. Tested fine on x86_64 and PPC64.

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

Commit messages:
 - Refactor IRIW tests a bit

Changes: https://git.openjdk.java.net/jcstress/pull/20/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jcstress&pr=20&range=00
  Stats: 224 lines in 4 files changed: 148 ins; 70 del; 6 mod
  Patch: https://git.openjdk.java.net/jcstress/pull/20.diff
  Fetch: git fetch https://git.openjdk.java.net/jcstress pull/20/head:pull/20

PR: https://git.openjdk.java.net/jcstress/pull/20