[ThinLTO] Avoid unnecesary hash lookups during metadata linking (NFC)

Replace sequences of count() followed by operator[] with either
find() or insert(), depending on the context.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@258405 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Bitcode/Reader/BitcodeReader.cpp

@@ -3081,9 +3081,11 @@ void BitcodeReader::saveMetadataList(
     if (!OnlyTempMD || (N && N->isTemporary())) {
       // Will call this after materializing each function, in order to
       // handle remapping of the function's instructions/metadata.
+      auto IterBool = MetadataToIDs.insert(std::make_pair(MD, ID));
       // See if we already have an entry in that case.
-      if (OnlyTempMD && MetadataToIDs.count(MD)) {
-        assert(MetadataToIDs[MD] == ID && "Inconsistent metadata value id");
+      if (OnlyTempMD && !IterBool.second) {
+        assert(IterBool.first->second == ID &&
+               "Inconsistent metadata value id");
         continue;
       }
       if (N && N->isTemporary())
@@ -3091,7 +3093,6 @@ void BitcodeReader::saveMetadataList(
         // metadata while it is the key of a map. The flag will be set back
         // to true when the saved metadata list is destroyed.
         N->setCanReplace(false);
-      MetadataToIDs[MD] = ID;
     }
   }
 }