DomTree: Make PostDomTree immune to block successors swap
This is another fix for the bug 46098 where PostDominatorTree
is unexpectedly changed by InstCombine's branch swapping
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