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.cc b/compiler/optimizing/scheduler.cc
index 9acf6d2..c673d54 100644
--- a/compiler/optimizing/scheduler.cc
+++ b/compiler/optimizing/scheduler.cc
@@ -564,7 +564,7 @@
 }
 
 void HScheduler::Schedule(HBasicBlock* block) {
-  ScopedArenaVector<SchedulingNode*> scheduling_nodes(arena_->Adapter(kArenaAllocScheduler));
+  ScopedArenaVector<SchedulingNode*> scheduling_nodes(allocator_->Adapter(kArenaAllocScheduler));
 
   // Build the scheduling graph.
   scheduling_graph_.Clear();
@@ -595,7 +595,7 @@
     }
   }
 
-  ScopedArenaVector<SchedulingNode*> initial_candidates(arena_->Adapter(kArenaAllocScheduler));
+  ScopedArenaVector<SchedulingNode*> initial_candidates(allocator_->Adapter(kArenaAllocScheduler));
   if (kDumpDotSchedulingGraphs) {
     // Remember the list of initial candidates for debug output purposes.
     initial_candidates.assign(candidates_.begin(), candidates_.end());