Convert diamond shaped control flow into a single conditional instruction.

runtime/vm/flow_graph_type_propagator.cc

Index: runtime/vm/flow_graph_type_propagator.cc
diff --git a/runtime/vm/flow_graph_type_propagator.cc b/runtime/vm/flow_graph_type_propagator.cc
index 52fd2e2c5fec273b06305e49a10a1497923e5299..34cef087a342dbca0e83075fb5350947a72aa247 100644
--- a/runtime/vm/flow_graph_type_propagator.cc
+++ b/runtime/vm/flow_graph_type_propagator.cc
@@ -645,6 +645,12 @@ bool PhiInstr::RecomputeType() {
 }
 
 
+CompileType IfThenElseInstr::ComputeType() const {
+  ASSERT(InputCount() == 2);
+  return CompileType::FromCid(kSmiCid);
+}
+
+
 CompileType ParameterInstr::ComputeType() const {
   // Note that returning the declared type of the formal parameter would be
   // incorrect, because ParameterInstr is used as input to the type check