Optimize range-delete aggregator call in merge helper.

Summary:
In the condition:
```
if (range_del_agg != nullptr &&
    range_del_agg->ShouldDelete(
        iter->key(),
        RangeDelAggregator::RangePositioningMode::kForwardTraversal) &&
    filter != CompactionFilter::Decision::kRemoveAndSkipUntil) {
...
}
```
it could be possible that all the work done in `range_del_agg->ShouldDelete` is wasted due to not having the right `filter` value later on.
Instead, check `filter` value before even calling `range_del_agg->ShouldDelete`, which is a much more involved function.
Closes https://github.com/facebook/rocksdb/pull/2690

Differential Revision: D5568931

Pulled By: sagar0

fbshipit-source-id: 17512d52360425c7ae9de7675383f5d7bc3dad58

db/merge_helper.cc

@@ -201,12 +201,11 @@ Status MergeHelper::MergeUntil(InternalIterator* iter,
           ikey.sequence <= latest_snapshot_
               ? CompactionFilter::Decision::kKeep
               : FilterMerge(orig_ikey.user_key, value_slice);
-      if (range_del_agg != nullptr &&
-
+      if (filter != CompactionFilter::Decision::kRemoveAndSkipUntil &&
+          range_del_agg != nullptr &&
           range_del_agg->ShouldDelete(
               iter->key(),
-              RangeDelAggregator::RangePositioningMode::kForwardTraversal) &&
-          filter != CompactionFilter::Decision::kRemoveAndSkipUntil) {
+              RangeDelAggregator::RangePositioningMode::kForwardTraversal)) {
         filter = CompactionFilter::Decision::kRemove;
       }
       if (filter == CompactionFilter::Decision::kKeep ||