Ignore definitions without uses in FlowGraphAllocator::CollectRepresentations.

runtime/vm/flow_graph_allocator.cc

Index: runtime/vm/flow_graph_allocator.cc
diff --git a/runtime/vm/flow_graph_allocator.cc b/runtime/vm/flow_graph_allocator.cc
index 4adf6818eacdb2cb23837d424a96959e96c27dc9..a2b0c3688c2f557017caaa5a08384a91ccec597a 100644
--- a/runtime/vm/flow_graph_allocator.cc
+++ b/runtime/vm/flow_graph_allocator.cc
@@ -2432,9 +2432,11 @@ void FlowGraphAllocator::CollectRepresentations() {
     for (ForwardInstructionIterator instr_it(block);
          !instr_it.Done();
          instr_it.Advance()) {
-      Instruction* instr = instr_it.Current();
-      if (instr->IsDefinition() && instr->representation() == kUnboxedMint) {
-        mint_values_->Add(instr->AsDefinition()->ssa_temp_index());
+      Definition* defn = instr_it.Current()->AsDefinition();
+      if ((defn != NULL) &&
+          (defn->ssa_temp_index() >= 0) &&
+          (defn->representation() == kUnboxedMint)) {
+        mint_values_->Add(defn->ssa_temp_index());
       }
     }
   }