8176565: G1 Does not perform heap verification after remark with VerifyAfterGC

[Mikael Gerdin]

Hi all,

Please review this small change to when G1 performs heap verification.

Background to this change is that it is fairly common knowledge that in 
order to rule out GC bugs you should run with -XX:+VerifyBeforeGC 
-XX:+VerifyAfterGC.
In order to make it easier for people to get the kind of heap 
verification they expect we should have G1 perform a verification at the 
end of the remark phase similar to what CMS does.
Previously this verification has only been enabled with 
-XX:+VerifyDuringGC for G1 but that is a lesser known flag and I think 
we should try to make it easier for our users to get the verification 
passes they expect.

This is a low-risk fix and it will be good to have in 9.0 for debugging 
purposes.

Webrev: http://cr.openjdk.java.net/~mgerdin/8176565/webrev.0/
Bug: https://bugs.openjdk.java.net/browse/JDK-8176565
Testing: JPRT, local testing with -XX:+VerifyAfterGC

/Mikael