[Turbofan] New DCHECK to ensure no use is mutated when iterating through them

This CL adds a DCHECK to make the uses() function safer: we store what
the next pointer should be to comparing with the next use (from the 
current element in the linked list). This helps detect code which 
invalidates the use-list iterator, which would otherwise cause 
hard-to-debug errors.

Bug: 
Change-Id: I3875361809ffd55fb8be2cbb15af3250e3fd4c12
Reviewed-on: https://chromium-review.googlesource.com/596030Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Alexandre Talon <alexandret@google.com>
Cr-Commit-Position: refs/heads/master@{#47074}

src/compiler/node.h

@@ -63,7 +63,7 @@ class V8_EXPORT_PRIVATE Node final {
                                : inputs_.outline_->count_;
   }
 
-#if DEBUG
+#ifdef DEBUG
   void Verify();
 #define BOUNDS_CHECK(index)                                                  \
   do {                                                                       \
@@ -593,7 +593,14 @@ class Node::Uses::const_iterator final {
   typedef Node** pointer;
   typedef Node*& reference;
 
-  const_iterator(const const_iterator& other) : current_(other.current_) {}
+  const_iterator(const const_iterator& other)
+      : current_(other.current_)
+#ifdef DEBUG
+        ,
+        next_(other.next_)
+#endif
+  {
+  }
 
   Node* operator*() const { return current_->from(); }
   bool operator==(const const_iterator& other) const {
@@ -604,7 +611,13 @@ class Node::Uses::const_iterator final {
   }
   const_iterator& operator++() {
     DCHECK_NOT_NULL(current_);
+    // Checking no use gets mutated while iterating through them, a potential
+    // very tricky cause of bug.
     current_ = current_->next;
+#ifdef DEBUG
+    DCHECK_EQ(current_, next_);
+    next_ = current_ ? current_->next : nullptr;
+#endif
     return *this;
   }
   const_iterator operator++(int);
@@ -613,9 +626,19 @@ class Node::Uses::const_iterator final {
   friend class Node::Uses;
 
   const_iterator() : current_(nullptr) {}
-  explicit const_iterator(Node* node) : current_(node->first_use_) {}
+  explicit const_iterator(Node* node)
+      : current_(node->first_use_)
+#ifdef DEBUG
+        ,
+        next_(current_ ? current_->next : nullptr)
+#endif
+  {
+  }
 
   Node::Use* current_;
+#ifdef DEBUG
+  Node::Use* next_;
+#endif
 };