RFR (XS): Native Brooks ptr accesses can be constant-folded

[Aleksey Shipilev]

Hi,

When I did the bugfix for 16-byte object alignments, I had to make the
byte/word offsets non-constant and agreeing with allocated sizes --
otherwise we were crashing with reading garbage. But now I see there is
a little inconsistency in C2 that bumps the size based on offset, not
the size. When we fix that, we can get our constant offsets back! Native
disassembly for, say, ShenandoahBarrierSet::obj_equals looks much better
now.

Webrev:
  http://cr.openjdk.java.net/~shade/shenandoah/brooks-cnst-fold/webrev.01/

Testing:
 hs_gc_shenandoah (fastdebug)

Thanks,
-Aleksey