When computing reverse-CFG reverse-post-order, skip backedges, as

When computing reverse-CFG reverse-post-order, skip backedges, as
detected in the forward-CFG DFS. This prevents the reverse-CFG from
visiting blocks inside loops after blocks that dominate them in the
case where loops have multiple exits.

No testcase, because this fixes a bug which in practice only shows
up in a full optimizer run, due to the use-list order.

This fixes rdar://10422791 and others.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@146408 91177308-0d34-0410-b5e6-96231b3b80d8