[DAGCombiner] Add node to the worklist in topological order in parallelizeChainedStores

Summary: As per title.

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

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66659

llvm-svn: 370056