[eigen] Adding zeroes to sparse matrix alters minimum degree ordering

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I'm encountering an issue with Incomplete Cholesky.  I have a case where the factorization is successful for a particular matrix, but when I add zeroes to the matrix (that is, set entries in the matrix that were implicit zeroes to explicit zeroes) the factorization fails.  I don't completely understand why this happens, but I have observed that the permutation produced by AMDOrdering is different for the two cases (and that's a level of magic too dark for me to dig into directly).

I haven't produced a minimum working example (I'm only observing this on medium-sized systems at the moment, 83x83), but I will attempt to do so if necessary.  I'm hoping that someone may have an insight without it though.

Thank you!

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