[DAGCombine] Make sure combined nodes are added back to the worklist in topological order.

Currently, a node and its users are added back to the worklist in reverse topological order after it is combined. This diff changes that order to be topological. This is part of a larger migration to get the DAGCombiner to process nodes in topological order.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D127115