Disable read barriers in GC VisitReferences calls.

It's safe to disable read barriers in the CC collector's
VisitReferences calls. It speeds up the collector by some amount,
eg. Ritzperf EAAC GC time on N5 decreases by ~4.2%.

This is based on Mathieu Chartier's idea.

Bug: 12687968

Change-Id: I884dee7018f92761fad54269f4cd01db4fa82f2a
4 files changed