fix a tiny bug

src/codegen/regalloc.cpp

@@ -36,6 +36,8 @@ RegAllocator::no_reg_alloca(Value *v) {
         else if (instr->is_zext()) { // only alloca for true use
             bool alloc;
             ASSERT_CMPINST_USED_ONCE(instr);
+            if (instr->get_use_list().size() == 0)
+                return false;
             auto use_ins = dynamic_cast<Instruction *>(
                 instr->get_use_list().begin()->val_);
             // assert(use_ins != nullptr && "should only be instruction?");