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/utils/jni_macro_assembler_test.h b/compiler/utils/jni_macro_assembler_test.h
index ba95e21..34ab4c3 100644
--- a/compiler/utils/jni_macro_assembler_test.h
+++ b/compiler/utils/jni_macro_assembler_test.h
@@ -58,8 +58,8 @@
   explicit JNIMacroAssemblerTest() {}
 
   void SetUp() OVERRIDE {
-    arena_.reset(new ArenaAllocator(&pool_));
-    assembler_.reset(CreateAssembler(arena_.get()));
+    allocator_.reset(new ArenaAllocator(&pool_));
+    assembler_.reset(CreateAssembler(allocator_.get()));
     test_helper_.reset(
         new AssemblerTestInfrastructure(GetArchitectureString(),
                                         GetAssemblerCmdName(),
@@ -76,7 +76,7 @@
   void TearDown() OVERRIDE {
     test_helper_.reset();  // Clean up the helper.
     assembler_.reset();
-    arena_.reset();
+    allocator_.reset();
   }
 
   // Override this to set up any architecture-specific things, e.g., CPU revision.
@@ -140,7 +140,7 @@
   }
 
   ArenaPool pool_;
-  std::unique_ptr<ArenaAllocator> arena_;
+  std::unique_ptr<ArenaAllocator> allocator_;
   std::unique_ptr<Ass> assembler_;
   std::unique_ptr<AssemblerTestInfrastructure> test_helper_;