[DAGCombine] Add node in the worklist in topological order in CombineTo

This is part of an ongoing effort toward making DAGCombine process the nodes in topological order.

This is able to discover a couple of new optimizations, but also causes a couple of regression. I nevertheless chose to submit this patch for review as to start the discussion with people working on the backend so we can find a good way forward.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D124743