Generalize getInvertibleOperand recurrence handling slightly

Follow up to D99912, specifically the revert, fix, and reapply thereof.

This generalizes the invertible recurrence logic in two ways:
* By allowing mismatching operand numbers of the phi, we can recurse through a pair of phi recurrences whose operand orders have not been canonicalized.
* By allowing recurrences through operand 1, we can invert these odd (but legal) recurrence.

Differential Revision: https://reviews.llvm.org/D100884