go/analysis/passes/nilness: fixed slice not being considered as non-nil

The slice produced by `s := []T{}` or `s := make([]T, 0)`
will always be non-nil. But, nilness could not detect it,
so I fixed it.

Fixes golang/go#45177

Author: komem3

go/analysis/passes/nilness/nilness.go

@@ -273,7 +273,8 @@ func nilnessOf(stack []fact, v ssa.Value) nilness {
 		*ssa.MakeClosure,
 		*ssa.MakeInterface,
 		*ssa.MakeMap,
-		*ssa.MakeSlice:
+		*ssa.MakeSlice,
+		*ssa.Slice:
 		return isnonnil
 	case *ssa.Const:
 		if v.IsNil() {

go/analysis/passes/nilness/testdata/src/a/a.go

@@ -71,6 +71,10 @@ func f3() error {
 	if ch != nil { // want "tautological condition: non-nil != nil"
 		print(0)
 	}
+	slice := make([]string, 0)
+	if slice == nil { // want "impossible condition: non-nil == nil"
+		print(0)
+	}
 	return nil
 }