RFR: 8257418: C2: Rename barrier data member in MemNode and LoadStoreNode

[Per Liden]

Please review this trivial cleanup, which renames the "barrier data" member in `MemNode` and `LoadStoreNode` from `_barrier` to `_barrier_data`, to better match the names of the setters and getters (`barrier_data()` and `set_barrier_data()`).

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Commit messages:
 - 8257418: C2: Rename barrier data member in MemNode and LoadStoreNode

Changes: https://git.openjdk.java.net/jdk/pull/1516/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1516&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257418
  Stats: 10 lines in 2 files changed: 0 ins; 0 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1516.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1516/head:pull/1516

PR: https://git.openjdk.java.net/jdk/pull/1516