[mlir][sparse] fix crash when calling getTuple on non-sparse tensors.

[Peiming]

Summary

This enables full sparse convolution codegen in D137298

[aartbik]

LGTM

mlir/lib/Dialect/SparseTensor/Transforms/SparseTensorCodegen.cpp

 /// Flatten a list of operands that may contain sparse tensors.
 static void flattenOperands(ValueRange operands,
                             SmallVectorImpl<Value> &flattened) {
   // In case of
   // sparse_tensor, c, sparse_tensor
   // ==>
   // memref ..., c, memref ...
   for (auto operand : operands) {
-    if (auto tuple = getTuple(operand);
-        tuple && getSparseTensorEncoding(tuple->getResultTypes()[0]))
+    if (getSparseTensorEncoding(operand.getType())) {
+      auto tuple = getTuple(operand);
       // An unrealized_conversion_cast will be inserted by type converter to
       // inter-mix the gap between 1:N conversion between sparse tensors and
       // fields. In this case, take the operands in the cast and replace the
       // sparse tensor output with the flattened type array.
       flattened.append(tuple.getOperands().begin(), tuple.getOperands().end());
-    else
+    } else {
       flattened.push_back(operand);
     }
   }
+}
 
 /// Adds index conversions where needed.
 static Value toType(OpBuilder &builder, Location loc, Value value, Type tp) {
   if (value.getType() != tp)
     return builder.create<arith::IndexCastOp>(loc, tp, value);
   return value;
 }