Use ScopedArenaAllocator for register allocation.

Memory needed to compile the two most expensive methods for
aosp_angler-userdebug boot image:
  BatteryStats.dumpCheckinLocked() : 25.1MiB -> 21.1MiB
  BatteryStats.dumpLocked(): 49.6MiB -> 42.0MiB
This is because all the memory previously used by Scheduler
is reused by the register allocator; the register allocator
has a higher peak usage of the ArenaStack.

And continue the "arena"->"allocator" renaming.

Test: m test-art-host-gtest
Test: testrunner.py --host
Bug: 64312607
Change-Id: Idfd79a9901552b5147ec0bf591cb38120de86b01
diff --git a/compiler/optimizing/codegen_test_utils.h b/compiler/optimizing/codegen_test_utils.h
index 5f4593f..bcbcc12 100644
--- a/compiler/optimizing/codegen_test_utils.h
+++ b/compiler/optimizing/codegen_test_utils.h
@@ -295,10 +295,15 @@
                            const std::function<void(HGraph*)>& hook_before_codegen,
                            bool has_result,
                            Expected expected) {
-  SsaLivenessAnalysis liveness(graph, codegen);
-  PrepareForRegisterAllocation(graph).Run();
-  liveness.Analyze();
-  RegisterAllocator::Create(graph->GetAllocator(), codegen, liveness)->AllocateRegisters();
+  {
+    ScopedArenaAllocator local_allocator(graph->GetArenaStack());
+    SsaLivenessAnalysis liveness(graph, codegen, &local_allocator);
+    PrepareForRegisterAllocation(graph).Run();
+    liveness.Analyze();
+    std::unique_ptr<RegisterAllocator> register_allocator =
+        RegisterAllocator::Create(&local_allocator, codegen, liveness);
+    register_allocator->AllocateRegisters();
+  }
   hook_before_codegen(graph);
   InternalCodeAllocator allocator;
   codegen->Compile(&allocator);