RFR: Verifier performance improvements: scan objects once, avoid double oop checks

[Aleksey Shipilev]

Marked object verification now takes considerable time, because it now touches
normally unreachable objects after TAMS. This captures most recent allocations,
especially during Init Mark verification, when immediate garbage regions are not
yet reclaimed.

We can improve verifier performance a little bit by using verification bitmap to
avoid scanning already scanned objects. We can also revise fully inline is_oop
assert to avoid useless double checking for is_in and check_alignment.

Webrev:
  http://cr.openjdk.java.net/~shade/shenandoah/verifier-perf/webrev.01/

Thanks,
-Aleksey