[turbofan] Update documentation for CompareOperationFeedback.

The CompareOperationFeedback documentation was outdated and there was an
invalid TODO on it that suggested to unify this with the
BinaryOperationFeedback which in retrospect doesn't make a lot of sense.

Bug: v8:7109
Change-Id: Ibf748e242db55430f29d305f1ef1df6d44449481
Reviewed-on: https://chromium-review.googlesource.com/819090Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49990}

src/globals.h

@@ -1279,11 +1279,14 @@ class BinaryOperationFeedback {
 // Type feedback is encoded in such a way that, we can combine the feedback
 // at different points by performing an 'OR' operation. Type feedback moves
 // to a more generic type when we combine feedback.
-// kSignedSmall        -> kNumber   -> kAny
+//
+//   kSignedSmall -> kNumber             -> kNumberOrOddball -> kAny
 //                   kInternalizedString -> kString          -> kAny
 //                                          kSymbol          -> kAny
 //                                          kReceiver        -> kAny
-// TODO(epertoso): consider unifying this with BinaryOperationFeedback.
+//
+// This is distinct from BinaryOperationFeedback on purpose, because the
+// feedback that matters differs greatly as well as the way it is consumed.
 class CompareOperationFeedback {
  public:
   enum {