cmd/compile: remove listsort

The listsort function is no longer used, except in a test.  Change the
test to use sort.Sort instead.

Change-Id: Ib634705cc1bc3b1d8fc3795bd4ed2894e6abc284
Reviewed-on: https://go-review.googlesource.com/19964
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/syntax.go

@@ -411,69 +411,6 @@ func list(l *NodeList, n *Node) *NodeList {
 	return concat(l, list1(n))
 }
 
-// listsort sorts *l in place according to the comparison function lt.
-// The algorithm expects lt(a, b) to be equivalent to a < b.
-// The algorithm is mergesort, so it is guaranteed to be O(n log n).
-func listsort(l **NodeList, lt func(*Node, *Node) bool) {
-	if *l == nil || (*l).Next == nil {
-		return
-	}
-
-	l1 := *l
-	l2 := *l
-	for {
-		l2 = l2.Next
-		if l2 == nil {
-			break
-		}
-		l2 = l2.Next
-		if l2 == nil {
-			break
-		}
-		l1 = l1.Next
-	}
-
-	l2 = l1.Next
-	l1.Next = nil
-	l2.End = (*l).End
-	(*l).End = l1
-
-	l1 = *l
-	listsort(&l1, lt)
-	listsort(&l2, lt)
-
-	if lt(l1.N, l2.N) {
-		*l = l1
-	} else {
-		*l = l2
-		l2 = l1
-		l1 = *l
-	}
-
-	// now l1 == *l; and l1 < l2
-
-	var le *NodeList
-	for (l1 != nil) && (l2 != nil) {
-		for (l1.Next != nil) && lt(l1.Next.N, l2.N) {
-			l1 = l1.Next
-		}
-
-		// l1 is last one from l1 that is < l2
-		le = l1.Next // le is the rest of l1, first one that is >= l2
-		if le != nil {
-			le.End = (*l).End
-		}
-
-		(*l).End = l1       // cut *l at l1
-		*l = concat(*l, l2) // glue l2 to *l's tail
-
-		l1 = l2 // l1 is the first element of *l that is < the new l2
-		l2 = le // ... because l2 now is the old tail of l1
-	}
-
-	*l = concat(*l, l2) // any remainder
-}
-
 // count returns the length of the list l.
 func count(l *NodeList) int {
 	n := int64(0)