cmd/compile: do not use "oaslit" for global

The compiler did not emit write barrier for assigning global with
struct literal, like global = T{} where T contains pointer.

The relevant code path is:
walkexpr OAS var_ OSTRUCTLIT
    oaslit
        anylit OSTRUCTLIT
            walkexpr OAS var_ nil
            return without adding write barrier
    return true
break (without adding write barrier)

This CL makes oaslit not apply to globals. See also CL
https://go-review.googlesource.com/c/36355/ for an alternative
fix.

The downside of this is that it generates static data for zeroing
struct now. Also this only covers global. If there is any lurking
bug with implicit zeroing other than globals, this doesn't fix.

Fixes #18956.

Change-Id: Ibcd27e4fae3aa38390ffa94a32a9dd7a802e4b37
Reviewed-on: https://go-review.googlesource.com/36410Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -585,7 +585,7 @@ func isliteral(n *Node) bool {
 }
 
 func (n *Node) isSimpleName() bool {
-	return n.Op == ONAME && n.Addable && n.Class != PAUTOHEAP
+	return n.Op == ONAME && n.Addable && n.Class != PAUTOHEAP && n.Class != PEXTERN
 }
 
 func litas(l *Node, r *Node, init *Nodes) {

test/writebarrier.go

@@ -220,3 +220,19 @@ func f22(x *int) (y *int) {
 	*p = x // no barrier
 	return
 }
+
+type T23 struct {
+	p *int
+	a int
+}
+
+var t23 T23
+var i23 int
+
+func f23() {
+	// zeroing global needs write barrier for the hybrid barrier.
+	t23 = T23{} // ERROR "write barrier"
+	// also test partial assignments
+	t23 = T23{a: 1}    // ERROR "write barrier"
+	t23 = T23{p: &i23} // ERROR "write barrier"
+}