RFR: Speed up asserts and verification, improve fastdebug builds performance

[Aleksey Shipilev]

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

This substantially improves ShenandoahAsserts performance, which translates to more GC cycles for
time-bound aggressive tests, and faster work-bound tests. This is achieved by carefully reshuffling
assert paths to avoid double/triple checks.

GCBasher/aggressive does 2x more GC cycles per minute with this patch.
hotspot_gc_shenandoah improves from 38 minutes to 26 minutes on my desktop.

Testing: hotspot_gc_shenandoah (fastdebug), hotspot_fast_gc_shenandoah (fastdebug|release)

Thanks,
-Aleksey