RFR: Add flag to switch between WB and RB-based acmp

[Roman Kennke]

This adds a flag to switch between WB- and RB-based acmp in C2. We'd
like to see if there's a measurable difference.

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

Testing: tier3_gc_shenandoah (with the flag turned on)

Roman