[SelectionDAG][DAGCombiner] Reuse exist node by reassociate

When already have (op N0, N2), reassociate (op (op N0, N1), N2) to (op (op N0, N2), N1) to reuse the exist (op N0, N2)

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D122539