IRBuilder: Ensure we do not add empty map elements

Do not use "Map[Key] == nullptr" to check if a Key is in the map, but use
"Map.find(Key) == Map.end()". Map[Key] always adds Key into the map, a
side-effect we do not want.

Found by inspection. This is hard to test outside of a targetted unit test,
which seems too much overhead for this individual issue.

llvm-svn: 249544