Review request: 8015486: PSScavenge::is_obj_in_young is unnecessarily slow with UseCompressedOops

[Stefan Karlsson]

http://cr.openjdk.java.net/~stefank/8015486/webrev.00/

8015486: PSScavenge::is_obj_in_young is unnecessarily slow with 
UseCompressedOops
Summary: Compare compressed oops to a compressed young gen boundary 
instead of uncompressing the oops before doing the young gen boundary check.

Instead of decompressing every single oop before checking against the 
young gen boundary, we could store a compressed young gen boundary and 
compare the compressed oops against that value. This will save both a 
null-check and the decompression code for all the visited oops.

This give about 3% lower young GC times when running SPECjbb2005.

StefanK