Use the proper post-order traversal in LiveVariables analysis,

Use the proper post-order traversal in LiveVariables analysis,
to recover the performance after r214064.

Also sorts out the naming for PostOrderCFGView, ReversePostOrderCFGView,
BackwardDataflowWorklist and ForwardDataflowWorklist, to match the accepted
terminology.

Also unifies BackwardDataflowWorklist and ForwardDataflowWorklist to share
the "worklist for prioritization, post-order traversal for fallback" logic,
and to avoid repetitive sorting.

Also cleans up comments in the affected area.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215650 91177308-0d34-0410-b5e6-96231b3b80d8