Use ScopedArenaAllocator for building HGraph.

Memory needed to compile the two most expensive methods for
aosp_angler-userdebug boot image:
  BatteryStats.dumpCheckinLocked() : 21.1MiB -> 20.2MiB
  BatteryStats.dumpLocked(): 42.0MiB -> 40.3MiB
This is because all the memory previously used by the graph
builder is reused by later passes.

And finish the "arena"->"allocator" renaming; make renamed
allocator pointers that are members of classes const when
appropriate (and make a few more members around them const).

Test: m test-art-host-gtest
Test: testrunner.py --host
Bug: 64312607
Change-Id: Ia50aafc80c05941ae5b96984ba4f31ed4c78255e
diff --git a/compiler/optimizing/scheduler.h b/compiler/optimizing/scheduler.h
index 3efd26a..afdf6f1 100644
--- a/compiler/optimizing/scheduler.h
+++ b/compiler/optimizing/scheduler.h
@@ -253,14 +253,14 @@
  public:
   SchedulingGraph(const HScheduler* scheduler, ScopedArenaAllocator* allocator)
       : scheduler_(scheduler),
-        arena_(allocator),
+        allocator_(allocator),
         contains_scheduling_barrier_(false),
-        nodes_map_(arena_->Adapter(kArenaAllocScheduler)),
+        nodes_map_(allocator_->Adapter(kArenaAllocScheduler)),
         heap_location_collector_(nullptr) {}
 
   SchedulingNode* AddNode(HInstruction* instr, bool is_scheduling_barrier = false) {
     std::unique_ptr<SchedulingNode> node(
-        new (arena_) SchedulingNode(instr, arena_, is_scheduling_barrier));
+        new (allocator_) SchedulingNode(instr, allocator_, is_scheduling_barrier));
     SchedulingNode* result = node.get();
     nodes_map_.Insert(std::make_pair(instr, std::move(node)));
     contains_scheduling_barrier_ |= is_scheduling_barrier;
@@ -323,7 +323,7 @@
 
   const HScheduler* const scheduler_;
 
-  ScopedArenaAllocator* const arena_;
+  ScopedArenaAllocator* const allocator_;
 
   bool contains_scheduling_barrier_;