Fix error in postfix ++ in Crankshaft.

src/ia32/lithium-ia32.cc

Index: src/ia32/lithium-ia32.cc
diff --git a/src/ia32/lithium-ia32.cc b/src/ia32/lithium-ia32.cc
index ee4f5c66826a5a8ebe1ba50630e6ec58c17d7ac5..f52ef9011b6185498d180063eb5685b5f5ab2168 100644
--- a/src/ia32/lithium-ia32.cc
+++ b/src/ia32/lithium-ia32.cc
@@ -1677,6 +1677,14 @@ 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();