Don't access parent of triangular matrix in powm (#52583)
Since the values stored in the parent corresponding to the structural
zeros of a tridiagonal matrix aren't well-defined, using it in `ldiv!`
is a footgun that may lead to heisenbugs (one seen in
https://buildkite.com/julialang/julia-master/builds/31285#018c7cc7-6c77-41ac-a01b-1c7d14cb1b15).
This PR changes it to using the tridiagonal matrix directly in `ldiv!`,
which should lead to predictable results, and be bug-free. The failing
tests for #52571 pass locally with this change.
