cmd/compile/internal/ssa: use reverse postorder traversal
Instead of the hand-written control flow analysis in debug info
generation, use a reverse postorder traversal, which is basically the
same thing. It should be slightly faster.
More importantly, the previous version simply gave up in the case of
non-reducible functions, and produced output that caused a later stage
to crash. It turns out that there's a non-reducible function in
compress/flate, so that wasn't a theoretical issue.
With this change, all blocks will be visited, even for non-reducible
functions.