Optimizing: Fix register allocator validation memory usage.

Also attribute ArenaBitVector allocations to appropriate
passes. This was used to track down the source of the
excessive memory alloactions.

Bug: 27690481

Change-Id: Ib895984cb7c04e24cbc7abbd8322079bab8ab100
diff --git a/runtime/base/scoped_arena_allocator.h b/runtime/base/scoped_arena_allocator.h
index a87153b..55044b3 100644
--- a/runtime/base/scoped_arena_allocator.h
+++ b/runtime/base/scoped_arena_allocator.h
@@ -152,6 +152,11 @@
   }
 
   template <typename T>
+  T* Alloc(ArenaAllocKind kind = kArenaAllocMisc) {
+    return AllocArray<T>(1, kind);
+  }
+
+  template <typename T>
   T* AllocArray(size_t length, ArenaAllocKind kind = kArenaAllocMisc) {
     return static_cast<T*>(Alloc(length * sizeof(T), kind));
   }