Fixed comparison in ForwardIterator when computing hint for GetNextLevelIndex()

Summary: When computing the hint for GetNextLevelIndex(), ForwardIterator was doing a redundant comparison. This patch fixes the comparison (using https://github.com/facebook/rocksdb/blob/master/db/version_set.cc#L158 as a reference) and moves it inside an assert because we expect `level_files[f_idx]` to contain the next key after Seek(), so user_key should always be smaller than the largest key.

Test Plan: make clean all check

Reviewers: rven, anthony, yhchiang, igor, sdong

Reviewed By: sdong

Subscribers: tnovak, sdong, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D46227

db/forward_iterator.cc

@@ -334,13 +334,9 @@ void ForwardIterator::SeekInternal(const Slice& internal_key,
         if (f_idx < level_files.size()) {
           int cmp_smallest = user_comparator_->Compare(
               user_key, level_files[f_idx]->smallest.user_key());
-          int cmp_largest = -1;
-          if (cmp_smallest >= 0) {
-            cmp_smallest = user_comparator_->Compare(
-                user_key, level_files[f_idx]->smallest.user_key());
-          }
-          indexer.GetNextLevelIndex(level, f_idx,
-              cmp_smallest, cmp_largest,
+          assert(user_comparator_->Compare(
+                     user_key, level_files[f_idx]->largest.user_key()) < 0);
+          indexer.GetNextLevelIndex(level, f_idx, cmp_smallest, -1,
                                     &search_left_bound, &search_right_bound);
         } else {
           indexer.GetNextLevelIndex(