Refactoring of graph linearization and linear order.

Rationale:
Ownership of graph's linear order and iterators was
a bit unclear now that other phases are using it.
New approach allows phases to compute their own
order, while ssa_liveness is sole owner for graph
(since it is not mutated afterwards).

Also shortens lifetime of loop's arena.

Test: test-art-host
Change-Id: Ib7137d1203a1e0a12db49868f4117d48a4277f30
diff --git a/runtime/base/arena_allocator.h b/runtime/base/arena_allocator.h
index 31dbb36..62cd2a7 100644
--- a/runtime/base/arena_allocator.h
+++ b/runtime/base/arena_allocator.h
@@ -80,6 +80,7 @@
   kArenaAllocDCE,
   kArenaAllocLSE,
   kArenaAllocLICM,
+  kArenaAllocLoopOptimization,
   kArenaAllocSsaLiveness,
   kArenaAllocSsaPhiElimination,
   kArenaAllocReferenceTypePropagation,