ConstantPropagation: make RemoveValidIf an optional dependency (#2027)

This allows ConstantPropagation to be used in cases where
ValidIfs need to be maintained, e.g., in the formal backend.

Co-authored-by: Adam Izraelevitz <adam.izraelevitz@sifive.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>