runtime: remove no-op pointer writes in treap rotations

Change-Id: If5a272f331fe9da09467efedd0231a4ce34db0f8
GitHub-Last-Rev: 4b81a79a92db4b51001ce6660b24c760fd3b630b
GitHub-Pull-Request: golang/go#28420
Reviewed-on: https://go-review.googlesource.com/c/go/+/144999
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/sema.go

@@ -373,19 +373,11 @@ Found:
 func (root *semaRoot) rotateLeft(x *sudog) {
 	// p -> (x a (y b c))
 	p := x.parent
-	a, y := x.prev, x.next
-	b, c := y.prev, y.next
+	y := x.next
+	b := y.prev
 
 	y.prev = x
 	x.parent = y
-	y.next = c
-	if c != nil {
-		c.parent = y
-	}
-	x.prev = a
-	if a != nil {
-		a.parent = x
-	}
 	x.next = b
 	if b != nil {
 		b.parent = x
@@ -409,23 +401,15 @@ func (root *semaRoot) rotateLeft(x *sudog) {
 func (root *semaRoot) rotateRight(y *sudog) {
 	// p -> (y (x a b) c)
 	p := y.parent
-	x, c := y.prev, y.next
-	a, b := x.prev, x.next
+	x := y.prev
+	b := x.next
 
-	x.prev = a
-	if a != nil {
-		a.parent = x
-	}
 	x.next = y
 	y.parent = x
 	y.prev = b
 	if b != nil {
 		b.parent = y
 	}
-	y.next = c
-	if c != nil {
-		c.parent = y
-	}
 
 	x.parent = p
 	if p == nil {