Fix error in postfix ++ in Crankshaft.

src/arm/lithium-arm.cc

Index: src/arm/lithium-arm.cc
diff --git a/src/arm/lithium-arm.cc b/src/arm/lithium-arm.cc
index 48c3ea39de6787a88e074916945ffb99d2ebfd7c..9387d90e4fa9c97789ea09dc066cbd6d1e6cad8a 100644
--- a/src/arm/lithium-arm.cc
+++ b/src/arm/lithium-arm.cc
@@ -1643,6 +1643,14 @@ LInstruction* LChunkBuilder::DoThrow(HThrow* instr) {
 }
 
 
+LInstruction* LChunkBuilder::DoForceRepresentation(HForceRepresentation* bad) {
+  // All HForceRepresentation instructions should be eliminated in the
+  // representation change phase of Hydrogen.
+  UNREACHABLE();
+  return NULL;
+}
+
+
 LInstruction* LChunkBuilder::DoChange(HChange* instr) {
   Representation from = instr->from();
   Representation to = instr->to();