RFR: 8287312: G1: Early return on first failure in VerifyRegionClosure

[Albert Mingkun Yang]

Simple restructuring to match the intent of the comment.

Test: hotspot_gc

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Commit messages:
 - g1-verify-heap

Changes: https://git.openjdk.java.net/jdk/pull/8882/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8882&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287312
  Stats: 26 lines in 1 file changed: 10 ins; 11 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8882.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8882/head:pull/8882

PR: https://git.openjdk.java.net/jdk/pull/8882