[MLProgram] Avoid repeated hash lookups (NFC) (#108928)

Author: kazutakahirata

mlir/lib/Dialect/MLProgram/Transforms/PipelineGlobalOps.cpp

@@ -100,10 +100,8 @@ LogicalResult MLProgramPipelineGlobals::buildGlobalMap(ModuleOp module) {
     for (size_t i = 0; i < work.size(); ++i) {
       callableMap[work[i]]->walk([&](CallOpInterface call) {
         auto symbol = dyn_cast<SymbolRefAttr>(call.getCallableForCallee());
-        if (!visited.contains(symbol)) {
-          visited.insert(symbol);
+        if (visited.insert(symbol).second)
           work.push_back(symbol);
-        }
       });
 
       for (auto load : opLoadSymbols[work[i]])