RFR: Guard interpreter keep alive barrier with ShenandoahKeepAliveBarrier

[Roman Kennke]

One thing that I found in traversal GC work: with 
-ShenandoahKeepAliveBarrier we still generate some code in the 
interpreter that is only used for the keep-alive-barrier. This patch 
avoids this. I realize that this should require some better refactoring 
(to move more of that code into keep_alive_barrier() to begin with), but 
I suspect that can wait until upstream codegens arrive, then we need to 
refactor it (big time) anyway.

http://cr.openjdk.java.net/~rkennke/interpreter_keep_alive_barrier/webrev.00/

Tests: hotspot_gc_shenandoah

Ok?