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/utils/arm/assembler_arm_vixl.h b/compiler/utils/arm/assembler_arm_vixl.h
index 9c11fd3..0e73e6b 100644
--- a/compiler/utils/arm/assembler_arm_vixl.h
+++ b/compiler/utils/arm/assembler_arm_vixl.h
@@ -151,8 +151,8 @@
  private:
   class ArmException;
  public:
-  explicit ArmVIXLAssembler(ArenaAllocator* arena)
-      : Assembler(arena) {
+  explicit ArmVIXLAssembler(ArenaAllocator* allocator)
+      : Assembler(allocator) {
     // Use Thumb2 instruction set.
     vixl_masm_.UseT32();
   }