RFR: 8278207: G1: Tighten verification in G1ResetSkipCompactingClosure

[Albert Mingkun Yang]

Simple change of expanding the verification logic to mirror its source.

Test: tier1-5

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

Commit messages:
 - full-gc-compact-assert

Changes: https://git.openjdk.java.net/jdk/pull/6698/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6698&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278207
  Stats: 15 lines in 1 file changed: 10 ins; 1 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6698.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6698/head:pull/6698

PR: https://git.openjdk.java.net/jdk/pull/6698