[ExecutionEngine] Avoid repeated hash lookups (NFC)

Author: kazutakahirata

llvm/lib/ExecutionEngine/Orc/COFFPlatform.cpp

@@ -484,10 +484,8 @@ COFFPlatform::buildJDDepMap(JITDylib &JD) {
           }
           DM.push_back(KV.first);
           // Push unvisited entry.
-          if (!JDDepMap.count(KV.first)) {
+          if (JDDepMap.try_emplace(KV.first).second)
             Worklist.push_back(KV.first);
-            JDDepMap[KV.first] = {};
-          }
         }
       });
     }