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/ssa_liveness_analysis.cc b/compiler/optimizing/ssa_liveness_analysis.cc
index fd56601..9ab7a89 100644
--- a/compiler/optimizing/ssa_liveness_analysis.cc
+++ b/compiler/optimizing/ssa_liveness_analysis.cc
@@ -56,7 +56,7 @@
         instructions_from_ssa_index_.push_back(current);
         current->SetSsaIndex(ssa_index++);
         current->SetLiveInterval(
-            LiveInterval::MakeInterval(graph_->GetAllocator(), current->GetType(), current));
+            LiveInterval::MakeInterval(allocator_, current->GetType(), current));
       }
       current->SetLifetimePosition(lifetime_position);
     }
@@ -74,7 +74,7 @@
         instructions_from_ssa_index_.push_back(current);
         current->SetSsaIndex(ssa_index++);
         current->SetLiveInterval(
-            LiveInterval::MakeInterval(graph_->GetAllocator(), current->GetType(), current));
+            LiveInterval::MakeInterval(allocator_, current->GetType(), current));
       }
       instructions_from_lifetime_position_.push_back(current);
       current->SetLifetimePosition(lifetime_position);
@@ -88,8 +88,8 @@
 
 void SsaLivenessAnalysis::ComputeLiveness() {
   for (HBasicBlock* block : graph_->GetLinearOrder()) {
-    block_infos_[block->GetBlockId()] = new (graph_->GetAllocator()) BlockInfo(
-        graph_->GetAllocator(), *block, number_of_ssa_values_);
+    block_infos_[block->GetBlockId()] =
+        new (allocator_) BlockInfo(allocator_, *block, number_of_ssa_values_);
   }
 
   // Compute the live ranges, as well as the initial live_in, live_out, and kill sets.