Make the code of blockEndsInUnreachable to match the function
blockEndsInUnreachable in CodeGen/BranchFolding.cpp. I also have
added a note to make sure the code of this function will not be
modified unless the back-end version is also modified.
We used to mark the whole function as cold by back-propagation
because the back-propagation would mark blocks with return
statements as cold.
The patch adds debug statements to help discover these problems.