RFR (S) 8190891: Clean up G1 barrier code in compiler interface (ci)

[coleen.phillimore at oracle.com]

Summary: consolidate gc barrier code in ci

See bug and comments for more details.

Tested with mach5 tier1-5 on linux x64 and iwndows x64.  Also tested 
StressRedefineWithoutBytecodeModification in 
test/jdk/java/lang/instrument which fails without the g1 barriers.

open webrev at http://cr.openjdk.java.net/~coleenp/8190891.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8190891

Thanks,
Coleen