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.

Change-Id: Id47536764ee93203c6b4105a1a3013fe3265aa12
Reviewed-on: https://go-review.googlesource.com/73110
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/fmt_test.go

@@ -606,7 +606,6 @@ var knownFormats = map[string]string{
 	"[16]byte %x":                                     "",
 	"[]*cmd/compile/internal/gc.Node %v":              "",
 	"[]*cmd/compile/internal/gc.Sig %#v":              "",
-	"[]*cmd/compile/internal/ssa.Block %+v":           "",
 	"[]*cmd/compile/internal/ssa.Value %v":            "",
 	"[][]cmd/compile/internal/ssa.SlotID %v":          "",
 	"[]byte %s":                                       "",