RFR: 8178836: Improve PtrQueue index abstraction

[Kim Barrett]

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