RFR (XXS): 8026293: Schedule part of G1 pre-barrier late

[Vladimir Kozlov]

http://cr.openjdk.java.net/~kvn/8026293/webrev/

https://bugs.openjdk.java.net/browse/JDK-8026293

A large part of the G1 write barrier is only used during marking, hence 
the code it not commonly executed.
To make the active code segments smaller and better fit the cache these 
unused segments should be schedule outside of the common path.
Setting the the first branch of the pre-barrier check to be unlikely 
will make the compiler treat the barrier code as cold and hence schedule 
it outside of the hot path.

Contributed by Staffan Friberg.

Tested JPRT, jtreg, SPECJEnt, SPECJBB.

Thanks,
Vladimir