[DAGCombiner] Remove a bunch of redundant AddToWorklist calls.

Summary:
This comes as a first step toward processing the DAG nodes in topological orders. Doing so ensure that arguments of a node are combined before the node itself is combined, which exposes ore opportunities for optimization and/or reduce the amount of patterns a node has to match for.

DAGCombiner adding nodes to the worklist is various places causes the nodes to be in a different order from what is expected. In addition, this is reduant because these nodes end up being added to the worklist anyways due to the machinery at line 1621.

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

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66537

llvm-svn: 369927