[sledge] Factor out solving polynomial = 0 equalities from solve
[sledge] Factor out solving polynomial = 0 equalities from solve

Summary:
Identifying and separating one of the monomials in a polynomial, and
solving an equality for it, is much more dependent on the
representation of polynomial terms than the rest of solve.

Reviewed By: ngorogiannis

Differential Revision: D19282645

fbshipit-source-id: 645191ae0

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