RFR: 8184914: Use MacroAssembler::cmpoop() consistently when comparing heap objects

[Roman Kennke]

(Not sure if this is the correct list to ask.. if not, please let me 
know and/or redirect me)

Currently, cmpoop() is only declared for 32-bit x86, and only used in 2 
places in C1 to compare oops. In other places, oops are compared using 
cmpptr(). It would be useful to distinguish normal pointer comparisons 
from heap object comparisons, and use cmpoop() consistently for heap 
object comparisons. This would remove clutter in several places where we 
have #ifdef _LP64 around comparisons, and would also allow to insert 
necessary barriers for GCs that need them (e.g. Shenandoah) later.

http://cr.openjdk.java.net/~rkennke/8184914/webrev.00/ 
<http://cr.openjdk.java.net/%7Erkennke/8184914/webrev.00/>

Tested by running hotspot_gc jtreg tests.

Can I get a review please?

Thanks, Roman