cmd/compile: avoid appends and allocate full slice length in copyRet

passes toolstash -cmp

compilebench allocs:
name      old allocs/op   new allocs/op   delta
Template       385k ± 0%       385k ± 0%  -0.00%  (p=0.017 n=19+20)
Unicode        342k ± 0%       342k ± 0%    ~     (p=0.867 n=20+20)
GoTypes       1.15M ± 0%      1.15M ± 0%  -0.00%  (p=0.008 n=20+20)
SSA           12.1M ± 0%      12.1M ± 0%    ~     (p=0.141 n=20+19)
Flate          234k ± 0%       234k ± 0%    ~     (p=0.125 n=20+19)
GoParser       315k ± 0%       315k ± 0%    ~     (p=0.104 n=20+20)
Reflect        972k ± 0%       972k ± 0%  -0.00%  (p=0.000 n=17+20)
Tar            391k ± 0%       391k ± 0%  -0.01%  (p=0.000 n=19+20)
XML            404k ± 0%       403k ± 0%  -0.01%  (p=0.000 n=20+19)

Change-Id: Ie24f7fae7b6b85422ec1ff0240f08f0a34064d72
Reviewed-on: https://go-review.googlesource.com/c/144038
Run-TryBot: Martin Möhrmann <martisch@uos.de>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -406,11 +406,13 @@ func (o *Order) copyRet(n *Node) []*Node {
 		Fatalf("copyret %v %d", n.Type, n.Left.Type.NumResults())
 	}
 
-	var l1, l2 []*Node
-	for _, f := range n.Type.Fields().Slice() {
-		tmp := temp(f.Type)
-		l1 = append(l1, tmp)
-		l2 = append(l2, tmp)
+	slice := n.Type.Fields().Slice()
+	l1 := make([]*Node, len(slice))
+	l2 := make([]*Node, len(slice))
+	for i, t := range slice {
+		tmp := temp(t.Type)
+		l1[i] = tmp
+		l2[i] = tmp
 	}
 
 	as := nod(OAS2, nil, nil)