RFR: JDK-8265394: G1: Improve assert in HeapRegion::reset_not_compacted_after_full_gc

[Hamlin Li]

This is a trivial assert message improvement/fix.

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Commit messages:
 - JDK-8265394: G1: Improve assert in HeapRegion::reset_not_compacted_after_full_gc

Changes: https://git.openjdk.java.net/jdk/pull/3680/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3680&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265394
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3680.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3680/head:pull/3680

PR: https://git.openjdk.java.net/jdk/pull/3680