cmd/compile: remove BlockDead state

It is unused, remove the clutter.

Change-Id: I51a44326b125ef79241459c463441f76a289cc08
Reviewed-on: https://go-review.googlesource.com/22586Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/ssa/check.go

@@ -85,19 +85,6 @@ func checkFunc(f *Func) {
 			if b.Aux == nil {
 				f.Fatalf("retjmp block %s has nil Aux field", b)
 			}
-		case BlockDead:
-			if len(b.Succs) != 0 {
-				f.Fatalf("dead block %s has successors", b)
-			}
-			if len(b.Preds) != 0 {
-				f.Fatalf("dead block %s has predecessors", b)
-			}
-			if len(b.Values) != 0 {
-				f.Fatalf("dead block %s has values", b)
-			}
-			if b.Control != nil {
-				f.Fatalf("dead block %s has a control value", b)
-			}
 		case BlockPlain:
 			if len(b.Succs) != 1 {
 				f.Fatalf("plain block %s len(Succs)==%d, want 1", b, len(b.Succs))

src/cmd/compile/internal/ssa/gen/genericOps.go

@@ -438,9 +438,8 @@ var genericBlocks = []blockData{
 	{name: "RetJmp"}, // no successors, jumps to b.Aux.(*gc.Sym)
 	{name: "Exit"},   // no successors, control value generates a panic
 
-	// transient block states used for dead code removal
+	// transient block state used for dead code removal
 	{name: "First"}, // 2 successors, always takes the first one (second is dead)
-	{name: "Dead"},  // no successors; determined to be dead but not yet removed
 }
 
 func init() {

src/cmd/compile/internal/ssa/opGen.go

@@ -47,7 +47,6 @@ const (
 	BlockRetJmp
 	BlockExit
 	BlockFirst
-	BlockDead
 )
 
 var blockString = [...]string{
@@ -88,7 +87,6 @@ var blockString = [...]string{
 	BlockRetJmp: "RetJmp",
 	BlockExit:   "Exit",
 	BlockFirst:  "First",
-	BlockDead:   "Dead",
 }
 
 func (k BlockKind) String() string { return blockString[k] }

src/cmd/compile/internal/ssa/rewrite.go

@@ -26,9 +26,6 @@ func applyRewrite(f *Func, rb func(*Block) bool, rv func(*Value, *Config) bool)
 	for {
 		change := false
 		for _, b := range f.Blocks {
-			if b.Kind == BlockDead {
-				continue
-			}
 			if b.Control != nil && b.Control.Op == OpCopy {
 				for b.Control.Op == OpCopy {
 					b.SetControl(b.Control.Args[0])