RFR: 8178836: Improve PtrQueue index abstraction

[Kim Barrett]

[oops, wrong list.  this was supposed to go to hotspot-gc-dev]

> On Apr 19, 2017, at 2:59 PM, Kim Barrett <kim.barrett at oracle.com> wrote:
> 
> Please review this API cleanup of PtrQueue and related classes.
> 
> PtrQueue internally represents buffer indices as byte offsets, so that
> generated code for queue insertion doesn't need to deal with scaling.
> 
> Most clients (other than the queue insertion code generators) don't
> need to know about that representation, and are simplified by dealing
> with indices and sizes in the natural units of an array of pointers.
> 
> This change eliminates the leakage of the byte offset representation
> to those other clients.
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8178836
> 
> Webrev:
> http://cr.openjdk.java.net/~kbarrett/8178836/hotspot.00/
> 
> Testing:
> JPRT
> rbt hs-tier2, hs-tier{3,4}-gc