[dev.ssa] Protect control value from being moved away from end of block

If there isn't a value dependency between the control value of a
block and some other value, the schedule pass might move the control
value to a spot that is not EOB. Fix by handling the control value
specially like phis.

Change-Id: Iddaf0924d98c5b3d9515c3ced927b0c85722818c
Reviewed-on: https://go-review.googlesource.com/11071Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -57,6 +57,9 @@ func schedule(f *Func) {
 		// Topologically sort the values in b.
 		order = order[:0]
 		for _, v := range b.Values {
+			if v == b.Control {
+				continue
+			}
 			if v.Op == OpPhi {
 				// Phis all go first.  We handle phis specially
 				// because they may have self edges "a = phi(a, b, c)"
@@ -79,13 +82,13 @@ func schedule(f *Func) {
 					// Note that v is not popped.  We leave it in place
 					// until all its children have been explored.
 					for _, w := range v.Args {
-						if w.Block == b && w.Op != OpPhi && state[w.ID] == unmarked {
+						if w.Block == b && w.Op != OpPhi && w != b.Control && state[w.ID] == unmarked {
 							state[w.ID] = found
 							queue = append(queue, w)
 						}
 					}
 					for _, w := range additionalEdges[v.ID] {
-						if w.Block == b && w.Op != OpPhi && state[w.ID] == unmarked {
+						if w.Block == b && w.Op != OpPhi && w != b.Control && state[w.ID] == unmarked {
 							state[w.ID] = found
 							queue = append(queue, w)
 						}
@@ -99,6 +102,9 @@ func schedule(f *Func) {
 				}
 			}
 		}
+		if b.Control != nil {
+			order = append(order, b.Control)
+		}
 		copy(b.Values, order)
 	}
 	// TODO: only allow one live flags type (x86)