[vm] Fix CheckForPointers to not ignore ranges of size 1.

TEST=ci
Change-Id: Id25b4ef224cd7c6ecf142b31311f51378366457d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/200529
Reviewed-by: Liam Appelbe <liama@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

runtime/vm/object.cc

@@ -18838,7 +18838,7 @@ class CheckForPointers : public ObjectPointerVisitor {
   bool has_pointers() const { return has_pointers_; }
 
   void VisitPointers(ObjectPtr* first, ObjectPtr* last) {
-    if (first != last) {
+    if (last >= first) {
       has_pointers_ = true;
     }
   }
@@ -18846,7 +18846,7 @@ class CheckForPointers : public ObjectPointerVisitor {
   void VisitCompressedPointers(uword heap_base,
                                CompressedObjectPtr* first,
                                CompressedObjectPtr* last) {
-    if (first != last) {
+    if (last >= first) {
       has_pointers_ = true;
     }
   }
@@ -18882,8 +18882,9 @@ void Instance::CanonicalizeFieldsLocked(Thread* thread) const {
   } else {
 #if defined(DEBUG)
     // Make sure that we are not missing any fields.
-    CheckForPointers has_pointers(IsolateGroup::Current());
-    this->ptr()->untag()->VisitPointers(&has_pointers);
+    IsolateGroup* group = IsolateGroup::Current();
+    CheckForPointers has_pointers(group);
+    this->ptr()->untag()->VisitPointersPrecise(group, &has_pointers);
     ASSERT(!has_pointers.has_pointers());
 #endif  // DEBUG
   }

runtime/vm/raw_object.cc

@@ -387,7 +387,7 @@ intptr_t UntaggedObject::VisitPointersPredefined(ObjectPointerVisitor* visitor,
 void UntaggedObject::VisitPointersPrecise(IsolateGroup* isolate_group,
                                           ObjectPointerVisitor* visitor) {
   intptr_t class_id = GetClassId();
-  if (class_id < kNumPredefinedCids) {
+  if ((class_id != kInstanceCid) && (class_id < kNumPredefinedCids)) {
     VisitPointersPredefined(visitor, class_id);
     return;
   }