Reuse sparsity without fixed-sparsity

[stevendargaville]

The high order non-fixed sparsity polynomial approximations don't currently reuse the symbolic matrix powers.

https://github.com/stevendargaville/PFLARE/blob/a8e2c404d2f21c79b82c77e4512929dfe87ca4bc/src/Gmres_Poly.F90#L1513C1-L1522C18