compiler: compare slice against nil

testdata/slice.go

@@ -4,6 +4,7 @@ func main() {
 	l := 5
 	foo := []int{1, 2, 4, 5}
 	bar := make([]int, l-2, l)
+	println("foo is nil?", foo == nil, nil == foo)
 	printslice("foo", foo)
 	printslice("bar", bar)
 	printslice("foo[1:2]", foo[1:2])
@@ -15,6 +16,7 @@ func main() {
 
 	// append
 	var grow []int
+	println("slice is nil?", grow == nil, nil == grow)
 	printslice("grow", grow)
 	grow = append(grow, 42)
 	printslice("grow", grow)

testdata/slice.txt

@@ -1,9 +1,11 @@
+foo is nil? false false
 foo: len=4 cap=4 data: 1 2 4 5
 bar: len=3 cap=5 data: 0 0 0
 foo[1:2]: len=1 cap=3 data: 2
 sum foo: 12
 copy foo -> bar: 3
 bar: len=3 cap=5 data: 1 2 4
+slice is nil? true true
 grow: len=0 cap=0 data:
 grow: len=1 cap=1 data: 42
 grow: len=3 cap=4 data: 42 -1 -2