RFR (M): Rehash barrier disable flags, add Shenandoah{CAS,Acmp,Clone}Barrier flags

[Aleksey Shipilev]

Hi,

Running with "passive" heuristics that frees us from requiring the operational
runtime barriers, and -XX:Shenandoah{Read,Write}Barriers are useful to dissect
runtime performance. However, those are handled inconsistently in the code, and
also we miss other three flavors of barriers: CAS, acmp, and clone barriers.'

This change rehashes the handling for these barrier disable flags:
  http://cr.openjdk.java.net/~shade/shenandoah/barrier-disable/webrev.01/

Disabling all barriers brings runtime performance very close to ParallelGC,
which seems to validate the approach.

Testing: hotspot_gc_shenandoah (x86), eyeballing assembly (x86), targeted
benchmarks (x86).

Thanks,
-Aleksey