Reorder dataflow to be earlier in fixed-point pipeline

Dataflow is capable of making large simplifications so its good to have it earlier if we can.

PiperOrigin-RevId: 698542726

xls/passes/optimization_pass_pipeline.cc

@@ -104,6 +104,8 @@ void AddSimplificationPasses(OptimizationCompoundPass& pass) {
   pass.Add<DeadCodeEliminationPass>();
   pass.Add<SelectSimplificationPass>();
   pass.Add<DeadCodeEliminationPass>();
+  pass.Add<DataflowSimplificationPass>();
+  pass.Add<DeadCodeEliminationPass>();
   pass.Add<LutConversionPass>();
   pass.Add<DeadCodeEliminationPass>();
   pass.Add<ConditionalSpecializationPass>(/*use_bdd=*/false);