Fix error in postfix ++ in Crankshaft.

src/x64/lithium-x64.cc

Index: src/x64/lithium-x64.cc
diff --git a/src/x64/lithium-x64.cc b/src/x64/lithium-x64.cc
index 00fd1bb8fc187d2e5f3a76e80ce87e365b073843..bfffc9cfe005cd51aca21bddfc30f861f4b26f56 100644
--- a/src/x64/lithium-x64.cc
+++ b/src/x64/lithium-x64.cc
@@ -1648,6 +1648,14 @@ 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();