When constant folding x == y take reachability of blocks into account.

runtime/vm/flow_graph_optimizer.h

Index: runtime/vm/flow_graph_optimizer.h
diff --git a/runtime/vm/flow_graph_optimizer.h b/runtime/vm/flow_graph_optimizer.h
index 3a98e8f21b5710ebb5e54f98dd5be747b7ec61f2..7ecb625fabe262a11950098ae6d01247ac37c9bd 100644
--- a/runtime/vm/flow_graph_optimizer.h
+++ b/runtime/vm/flow_graph_optimizer.h
@@ -345,7 +345,10 @@ class ConstantPropagator : public FlowGraphVisitor {
   void EliminateRedundantBranches();
 
   void SetReachable(BlockEntryInstr* block);
-  void SetValue(Definition* definition, const Object& value);
+  bool SetValue(Definition* definition, const Object& value);
+
+  Definition* UnwrapPhi(Definition* defn);
+  void MarkPhi(Definition* defn);
 
   // Assign the join (least upper bound) of a pair of abstract values to the
   // first one.
@@ -381,14 +384,11 @@ class ConstantPropagator : public FlowGraphVisitor {
   // preorder number.
   BitVector* reachable_;
 
-  // Definitions can move up the lattice twice, so we use a mark bit to
-  // indicate that they are already on the worklist in order to avoid adding
-  // them again.  Indexed by SSA temp index.
-  BitVector* definition_marks_;
+  BitVector* marked_phis_;
 
   // Worklists of blocks and definitions.
   GrowableArray<BlockEntryInstr*> block_worklist_;
-  GrowableArray<Definition*> definition_worklist_;
+  DefinitionWorklist definition_worklist_;
 };