A fix for the bug 46098 "PostDominatorTree is different than a freshly computed one!" with opt -branch-prob -instcombine -block-freq -verify-dom-info
InstCombine invalidates PostDomTree.
See the example in the new test llvm/test/Transforms/InstCombine/infinite-loop-postdom.ll
In general InstCombine swapping successors violates the 2nd rule of PreserveCFG: do not change block terminators in any way. Doing so InstCombine also invalidates DFS numbering, which is a part of DomTree.
This patch reflects these by reporting not preserving CFG.