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.

llvm-svn: 146408