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/code_generator_vector_arm64.cc b/compiler/optimizing/code_generator_vector_arm64.cc
index bcf35d9..10aced0 100644
--- a/compiler/optimizing/code_generator_vector_arm64.cc
+++ b/compiler/optimizing/code_generator_vector_arm64.cc
@@ -179,8 +179,8 @@
 }
 
 // Helper to set up locations for vector unary operations.
-static void CreateVecUnOpLocations(ArenaAllocator* arena, HVecUnaryOperation* instruction) {
-  LocationSummary* locations = new (arena) LocationSummary(instruction);
+static void CreateVecUnOpLocations(ArenaAllocator* allocator, HVecUnaryOperation* instruction) {
+  LocationSummary* locations = new (allocator) LocationSummary(instruction);
   switch (instruction->GetPackedType()) {
     case DataType::Type::kBool:
       locations->SetInAt(0, Location::RequiresFpuRegister());
@@ -372,8 +372,8 @@
 }
 
 // Helper to set up locations for vector binary operations.
-static void CreateVecBinOpLocations(ArenaAllocator* arena, HVecBinaryOperation* instruction) {
-  LocationSummary* locations = new (arena) LocationSummary(instruction);
+static void CreateVecBinOpLocations(ArenaAllocator* allocator, HVecBinaryOperation* instruction) {
+  LocationSummary* locations = new (allocator) LocationSummary(instruction);
   switch (instruction->GetPackedType()) {
     case DataType::Type::kBool:
     case DataType::Type::kUint8:
@@ -772,8 +772,8 @@
 }
 
 // Helper to set up locations for vector shift operations.
-static void CreateVecShiftLocations(ArenaAllocator* arena, HVecBinaryOperation* instruction) {
-  LocationSummary* locations = new (arena) LocationSummary(instruction);
+static void CreateVecShiftLocations(ArenaAllocator* allocator, HVecBinaryOperation* instruction) {
+  LocationSummary* locations = new (allocator) LocationSummary(instruction);
   switch (instruction->GetPackedType()) {
     case DataType::Type::kUint8:
     case DataType::Type::kInt8:
@@ -967,8 +967,8 @@
 }
 
 // Helper to set up locations for vector accumulations.
-static void CreateVecAccumLocations(ArenaAllocator* arena, HVecOperation* instruction) {
-  LocationSummary* locations = new (arena) LocationSummary(instruction);
+static void CreateVecAccumLocations(ArenaAllocator* allocator, HVecOperation* instruction) {
+  LocationSummary* locations = new (allocator) LocationSummary(instruction);
   switch (instruction->GetPackedType()) {
     case DataType::Type::kUint8:
     case DataType::Type::kInt8:
@@ -1216,10 +1216,10 @@
 }
 
 // Helper to set up locations for vector memory operations.
-static void CreateVecMemLocations(ArenaAllocator* arena,
+static void CreateVecMemLocations(ArenaAllocator* allocator,
                                   HVecMemoryOperation* instruction,
                                   bool is_load) {
-  LocationSummary* locations = new (arena) LocationSummary(instruction);
+  LocationSummary* locations = new (allocator) LocationSummary(instruction);
   switch (instruction->GetPackedType()) {
     case DataType::Type::kBool:
     case DataType::Type::kUint8: