Re-enable most intrinsics with read barriers.

Also extend sun.misc.Unsafe test coverage to exercise
sun.misc.Unsafe.{get,put}{Int,Long,Object}Volatile.

Bug: 26205973
Bug: 29516905
Change-Id: I4d8da7cee5c8a310c8825c1631f71e5cb2b80b30
Test: Covered by ART's run-tests.
diff --git a/compiler/optimizing/locations.h b/compiler/optimizing/locations.h
index 63bbc2c..3f27c91 100644
--- a/compiler/optimizing/locations.h
+++ b/compiler/optimizing/locations.h
@@ -38,7 +38,13 @@
 class Location : public ValueObject {
  public:
   enum OutputOverlap {
+    // The liveness of the output overlaps the liveness of one or
+    // several input(s); the register allocator cannot reuse an
+    // input's location for the output's location.
     kOutputOverlap,
+    // The liveness of the output does not overlap the liveness of any
+    // input; the register allocator is allowed to reuse an input's
+    // location for the output's location.
     kNoOutputOverlap
   };
 
@@ -494,6 +500,10 @@
     return inputs_.size();
   }
 
+  // Set the output location.  Argument `overlaps` tells whether the
+  // output overlaps any of the inputs (if so, it cannot share the
+  // same register as one of the inputs); it is set to
+  // `Location::kOutputOverlap` by default for safety.
   void SetOut(Location location, Location::OutputOverlap overlaps = Location::kOutputOverlap) {
     DCHECK(output_.IsInvalid());
     output_overlaps_ = overlaps;