RFR(XS): 8015244: G1: Verification after a full GC is incorrectly placed

[John Cuthbertson]

Hi Everyone,

Can I have a couple of volunteers look over this small fix? The webrev 
can be found at: http://cr.openjdk.java.net/~johnc/8015244/webrev.0/

Summary:
While investigating some additional verification code for "7145569: G1: 
optimize nmethods scanning", I realized that the verification that 
occurs after a full GC is misplaced. I think it should come after the 
rebuilding of the RSets and verify the rebuilt RSets. Verifying the 
RSets before the full GC is less interesting and potentially wasteful so 
I placed the RSet verification during full GCs under control of an 
additional diagnostic flag.

These aren't true performance numbers but when I ran GCOld with the new 
flag enabled (1G heap), I saw an increase in full GC times of around 4% 
on my workstation (VerifyBefore and VerifyAfter were both enabled).

Testing:
GCOld; CG test suite

Thanks,

JohnC