RFR (XS): 8200362: G1Mux2Closure should disable implicit oop verification

[Thomas Schatzl]

Hi all,

  can I have reviews for this small change that fixes some annoyance
when debugging with G1?

In particular, G1Mux2Closure is used by verification to verify and in
case of error print out g1 specific error messages. 

However, since G1Mux2Closure is an OopClosure, it is actually wrapped
by NoHeaderExtendedOopClosure in oop_iterate* calls. ExtendedOopClosure
has its own verification that is very generic, and may trigger in the
same situations as G1Mux2Closure. 

Disable this implicit verification for G1Mux2Closure so that we always
get the g1 specific error messages.

CR:
https://bugs.openjdk.java.net/browse/JDK-8200362
Webrev:
http://cr.openjdk.java.net/~tschatzl/8200362/webrev/
Testing:
(mostly finished) hs-tier 1+2

Thanks,
  Thomas