[dev.ssa] cmd/compile: schedule values dependent on the control later

To reduce the number of spills, give any non-phi values whose argument
is the control the same priority as the control.

With mask.bash, this reduces regenerated flags from 603 to 240.

Change-Id: I26883d69e80357c56b343428fb528102b3f26e7a
Reviewed-on: https://go-review.googlesource.com/14042Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -89,9 +89,21 @@ func schedule(f *Func) {
 			// Force the control value to be scheduled at the end,
 			// unless it is a phi value (which must be first).
 			score[b.Control.ID] = 4
-			// TODO: some times control values are used by other values
-			// in the block.  So the control value will not appear at
-			// the very end.  Decide if this is a problem or not.
+
+			// Schedule values dependent on the control value at the end.
+			// This reduces the number of register spills. We don't find
+			// all values that depend on the control, just values with a
+			// direct dependency.  This is cheaper and in testing there
+			// was no difference in the number of spills.
+			for _, v := range b.Values {
+				if v.Op != OpPhi {
+					for _, a := range v.Args {
+						if a == b.Control {
+							score[v.ID] = 4
+						}
+					}
+				}
+			}
 		}
 
 		// Initialize priority queue with schedulable values.