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/scheduler_arm64.h b/compiler/optimizing/scheduler_arm64.h
index 6682c66..32f161f 100644
--- a/compiler/optimizing/scheduler_arm64.h
+++ b/compiler/optimizing/scheduler_arm64.h
@@ -131,8 +131,8 @@
 
 class HSchedulerARM64 : public HScheduler {
  public:
-  HSchedulerARM64(ScopedArenaAllocator* arena, SchedulingNodeSelector* selector)
-      : HScheduler(arena, &arm64_latency_visitor_, selector) {}
+  HSchedulerARM64(ScopedArenaAllocator* allocator, SchedulingNodeSelector* selector)
+      : HScheduler(allocator, &arm64_latency_visitor_, selector) {}
   ~HSchedulerARM64() OVERRIDE {}
 
   bool IsSchedulable(const HInstruction* instruction) const OVERRIDE {