Replace memory barriers to better reflect Java needs.

Replaces barriers that enforce ordering of one access type
(e.g. Load) with respect to another (e.g. store) with more general
ones that better reflect both Java requirements and actual hardware
barrier/fence instructions.  The old code was inconsistent and
unclear about which barriers implied which others.  Sometimes
multiple barriers were generated and then eliminated;
sometimes it was assumed that certain barriers implied others.
The new barriers closely parallel those in C++11, though, for now,
we use something closer to the old naming.

Bug: 14685856

Change-Id: Ie1c80afe3470057fc6f2b693a9831dfe83add831