[SCEV] Generalize MatchBinaryAddToConst to support non-add expressions.

This patch generalizes MatchBinaryAddToConst to support matching
(A + C1), (A + C2), instead of just matching (A + C1), A.

The existing cases can be handled by treating non-add expressions A as
A + 0.

Reviewed By: mkazantsev

Differential Revision: https://reviews.llvm.org/D104634