[mlir][reducer] make opt-reduction pass clone topOp after check (NFC) (#189356)
To avoid potential memory leaks, this PR defers the ModuleOp cloning
until after the verification check. If the check fails, the
moduleVariant might not be properly deallocated(original
implementation), leading to a memory leak. Therefore, this PR ensures
that the clone operation is only performed after a successful check. It
is part of https://github.com/llvm/llvm-project/pull/189353.