Subzero. Liveness memory management.

Creates a local arena allocator for holding liveness data structures.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4366
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1838973005 .
diff --git a/src/IceLiveness.cpp b/src/IceLiveness.cpp
index 46c834b..8990a3e 100644
--- a/src/IceLiveness.cpp
+++ b/src/IceLiveness.cpp
@@ -106,8 +106,8 @@
       RangeMask[VarIndex] = false;
   }
 
-  SizeT MaxLocals = 0;
   // Process each node.
+  MaxLocals = 0;
   for (auto I = FirstNode, E = Func->getNodes().end(); I != E; ++I) {
     LivenessNode &Node = Nodes[(*I)->getIndex()];
     // NumLocals, LiveToVarMap already initialized