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/intrinsics_mips64.h b/compiler/optimizing/intrinsics_mips64.h
index 6085c7b..6f40d90 100644
--- a/compiler/optimizing/intrinsics_mips64.h
+++ b/compiler/optimizing/intrinsics_mips64.h
@@ -49,8 +49,8 @@
   bool TryDispatch(HInvoke* invoke);
 
  private:
-  CodeGeneratorMIPS64* codegen_;
-  ArenaAllocator* allocator_;
+  CodeGeneratorMIPS64* const codegen_;
+  ArenaAllocator* const allocator_;
 
   DISALLOW_COPY_AND_ASSIGN(IntrinsicLocationsBuilderMIPS64);
 };
@@ -73,7 +73,7 @@
 
   ArenaAllocator* GetAllocator();
 
-  CodeGeneratorMIPS64* codegen_;
+  CodeGeneratorMIPS64* const codegen_;
 
   DISALLOW_COPY_AND_ASSIGN(IntrinsicCodeGeneratorMIPS64);
 };