Fix Node::ReplaceUses to handle unused nodes.

src/compiler/generic-node-inl.h

Index: src/compiler/generic-node-inl.h
diff --git a/src/compiler/generic-node-inl.h b/src/compiler/generic-node-inl.h
index e7a8f90f61b0726036df1caddbc23492ff21e9b3..82fd907c16e18811c00b8bf2c2ab3e4d9a5b2903 100644
--- a/src/compiler/generic-node-inl.h
+++ b/src/compiler/generic-node-inl.h
@@ -64,12 +64,13 @@ void GenericNode<B, S>::ReplaceUses(GenericNode* replace_to) {
   if (replace_to->last_use_ == NULL) {
     DCHECK_EQ(NULL, replace_to->first_use_);
     replace_to->first_use_ = first_use_;
-  } else {
+    replace_to->last_use_ = last_use_;
+  } else if (first_use_ != NULL) {
     DCHECK_NE(NULL, replace_to->first_use_);
     replace_to->last_use_->next = first_use_;
     first_use_->prev = replace_to->last_use_;
+    replace_to->last_use_ = last_use_;
   }
-  replace_to->last_use_ = last_use_;
   replace_to->use_count_ += use_count_;
   use_count_ = 0;
   first_use_ = NULL;