RFR: Hook up Shenandoah barrier expand to ShBSC2::expand_macro_nodes()

[Roman Kennke]

As $SUBJECT says. Some comments:
- Instead of using the global loop_opts_cnt, it uses a local
loop_opts_cnt. That means, even if global loop opts have been depleted,
we still give it 7 (default value) more tries.
- Compile* C and PhaseIGVN* igvn is now fetched from PhaseMacroExpand*.

http://cr.openjdk.java.net/~rkennke/c2-barrier-expand/webrev.00/

Testing: tier3_gc_shenandoah, I intend to run a few benchmarks too

Thoughts? Did I miss something?

Roman