[DAGCombiner] Add node back in the worklist in topological order in CommitTargetLoweringOpt

Summary:
Right now, DAGCombiner process the nodes in an iplementation defined order. This tends to be fragile as optimisation may or may not kick in depending on the traversal order.

This is part of a larger effort to get the DAGCombiner to process its node in topological order.

Reviewers: craig.topper, efriedma, RKSimon, lebedev.ri

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70921