DomTree: Make PostDomTree indifferent to block successors swap

Fixed the commit c35585e2.

This is a fix for the bug 46098 where PostDominatorTree
is unexpectedly changed by InstCombine's branch swapping
transformation.
This patch fixes PostDomTree builder. While looking for
the furthest away node in a reverse unreachable subgraph
this patch runs DFS with successors in their function order.
This order is indifferent to the order of successors, so is
the furthest away node.

Reviewers: kuhar, nikic, lebedev.ri
Differential Revision: https://reviews.llvm.org/D84763