RFC: Accept embedded oops barriers to alleviate final mark time

[Aleksey Shipilev]

Hi,

This is my experimental patch for accepting barriers in the code cache, *and*
avoiding the evacuation work at the pause. It is dirty, and maybe incorrect, but
it passes hotspot_gc_shenandoah, and provides nice improvements for final mark
pause time. The overheads for emitting the barrier for constants is hard to
quantify before Roland fixes up the compiler bugs around it. Throwing the patch
out for the record, while I am going on vacation :)

Webrev:
  http://cr.openjdk.java.net/~shade/shenandoah/codecache-barriers/webrev.01/

GC stats before/after on some workloads:
  http://cr.openjdk.java.net/~shade/shenandoah/codecache-barriers/perf.txt

Init evac times, and sometimes final mark times are down to 100-500 us range!

Thanks,
-Aleksey