Begin migration of art::Atomic to std::atomic.

Change-Id: I4858d9cbed95e5ca560956b9dabd976cebe68333
diff --git a/compiler/driver/compiler_driver.cc b/compiler/driver/compiler_driver.cc
index eb62f1b..999d6a5 100644
--- a/compiler/driver/compiler_driver.cc
+++ b/compiler/driver/compiler_driver.cc
@@ -1369,7 +1369,7 @@
     self->AssertNoPendingException();
     CHECK_GT(work_units, 0U);
 
-    index_ = begin;
+    index_.StoreRelaxed(begin);
     for (size_t i = 0; i < work_units; ++i) {
       thread_pool_->AddTask(self, new ForAllClosure(this, end, callback));
     }
@@ -1384,7 +1384,7 @@
   }
 
   size_t NextIndex() {
-    return index_.FetchAndAdd(1);
+    return index_.FetchAndAddSequentiallyConsistent(1);
   }
 
  private: