StructurizeCFG: Use a reverse post-order traversal

We were previously doing a post-order traversal and operating on the
list in reverse, however this would occasionaly cause backedges for
loops to be visited before some of the other blocks in the loop.

We know use a reverse post-order traversal, which avoids this issue.

The reverse post-order traversal is not completely ideal, so we need
to manually fixup the list to ensure that inner loop backedges are
visited before outer loop backedges.

llvm-svn: 228186