Fix real schur and polynomial solver.

For certain inputs, the real schur decomposition might get stuck in a cycle.
Exceptional shifts are supposed to knock us out of that - but previously
they were only ever applied at iteration 10 and 30, which doesn't help if
the cycle starts after cycle 30.  Modified to apply a shift every 16 iterations
(for reference, LAPACK seems to do it every 6 iterations).

Also added an assert in polynomial solver to verify that the schur decomposition
was successful.

Fixes #2633.

unsupported/Eigen/src/Polynomials/PolynomialSolver.h

@@ -354,6 +354,7 @@ class PolynomialSolver : public PolynomialSolverBase<_Scalar,_Deg>
         internal::companion<Scalar,_Deg> companion( poly );
         companion.balance();
         m_eigenSolver.compute( companion.denseMatrix() );
+        eigen_assert(m_eigenSolver.info() == Eigen::Success);
         m_roots = m_eigenSolver.eigenvalues();
         // cleanup noise in imaginary part of real roots:
         // if the imaginary part is rather small compared to the real part