ART: Boolean simplifier

The optimization recognizes the negation pattern generated by 'javac'
and replaces it with a single condition. To this end, boolean values
are now consistently assumed to be represented by an integer.

This is a first optimization which deletes blocks from the HGraph and
does so by replacing the corresponding entries with null. Hence,
existing code can continue indexing the list of blocks with the block
ID, but must check for null when iterating over the list.

Change-Id: I7779da69cfa925c6521938ad0bcc11bc52335583
diff --git a/compiler/optimizing/ssa_liveness_analysis.cc b/compiler/optimizing/ssa_liveness_analysis.cc
index c0d6f42..56ccd71 100644
--- a/compiler/optimizing/ssa_liveness_analysis.cc
+++ b/compiler/optimizing/ssa_liveness_analysis.cc
@@ -71,8 +71,8 @@
   //      for it.
   GrowableArray<uint32_t> forward_predecessors(graph_.GetArena(), graph_.GetBlocks().Size());
   forward_predecessors.SetSize(graph_.GetBlocks().Size());
-  for (size_t i = 0, e = graph_.GetBlocks().Size(); i < e; ++i) {
-    HBasicBlock* block = graph_.GetBlocks().Get(i);
+  for (HReversePostOrderIterator it(graph_); !it.Done(); it.Advance()) {
+    HBasicBlock* block = it.Current();
     size_t number_of_forward_predecessors = block->GetPredecessors().Size();
     if (block->IsLoopHeader()) {
       // We rely on having simplified the CFG.