[mlir][pdl] OperationOp should not be side-effect free

Unbound OperationOp in the matcher (i.e. one with no uses) is already disallowed by the verifier. However, an OperationOp in the rewriter is not side-effect free -- it's creating an op!

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D117825