RFR: Reclaimed humongous regions should count towards immediate garbage

[Aleksey Shipilev]

With cycle shortcutting, it matters that we get this right. Humongous objects reclaimed before cset
construction should be counted as immediate garbage, so that we shortcut more cycles, otherwise we
do some cycles for almost nothing. For example, in some SPECjvm tests, the immediate garbage in
humongous regions is responsible for >5% of total garbage, tripping the shortcutting threshold with
this fix.

Webrev:
 http://cr.openjdk.java.net/~shade/shenandoah/immediate-humongous/webrev.01/

Testing: hotspot_gc_shenandoah, some benchmarks

Thanks,
-Aleksey