Change mlir::tf_saved_model::LookupBoundInput() to return mlir::Operation*.

Change mlir::tf_saved_model::LookupBoundInput() to return mlir::Operation* as there will be different kinds of operations for bound inputs. Also added a helper function LookupBoundInputOfType() to get bound inputs of the expected type.

PiperOrigin-RevId: 319999187
diff --git a/integrations/tensorflow/compiler/TFSavedModelLowerGlobalTensors.cpp b/integrations/tensorflow/compiler/TFSavedModelLowerGlobalTensors.cpp
index 6d8271d..03c878d 100644
--- a/integrations/tensorflow/compiler/TFSavedModelLowerGlobalTensors.cpp
+++ b/integrations/tensorflow/compiler/TFSavedModelLowerGlobalTensors.cpp
@@ -93,8 +93,8 @@
     OpBuilder builder(func.getBody());
     SmallVector<Value, 8> typeConversionWorklist;
     for (int i = 0, e = func.getNumArguments(); i < e; i++) {
-      auto globalTensor =
-          tf_saved_model::LookupBoundInput(func, i, symbolTable);
+      auto globalTensor = tf_saved_model::LookupBoundInputOfType<
+          tf_saved_model::GlobalTensorOp>(func, i, symbolTable);
       if (!globalTensor) {
         continue;
       }