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 752341b30ca86478de82a1ae03ef4881f657ef32..e9e8351a0e81c2cc7953ad1af1aec44f8004f7cb 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();