BlockBasedTable::PrefixMayMatch() to bloom setting to the beginning of the function

Summary: In BlockBasedTable::PrefixMayMatch() we calculate prefix even if bloom is not config. Move the check before

Test Plan: make all check

Reviewers: igor, ljin

Reviewed By: ljin

Subscribers: wuj, leveldb, haobo, yhchiang, dhruba

Differential Revision: https://reviews.facebook.net/D18993

table/block_based_table_reader.cc

@@ -914,6 +914,10 @@ class BlockBasedTable::BlockEntryIteratorState : public TwoLevelIteratorState {
 //
 // REQUIRES: this method shouldn't be called while the DB lock is held.
 bool BlockBasedTable::PrefixMayMatch(const Slice& internal_key) {
+  if (!rep_->options.filter_policy) {
+    return true;
+  }
+
   assert(rep_->options.prefix_extractor != nullptr);
   auto prefix = rep_->options.prefix_extractor->Transform(
       ExtractUserKey(internal_key));
@@ -923,10 +927,6 @@ bool BlockBasedTable::PrefixMayMatch(const Slice& internal_key) {
   bool may_match = true;
   Status s;
 
-  if (!rep_->options.filter_policy) {
-    return true;
-  }
-
   // To prevent any io operation in this method, we set `read_tier` to make
   // sure we always read index or filter only when they have already been
   // loaded to memory.