cmd/compile: lower slice clears to memclrHasPointers

If a slice's backing store has pointers, we need to lower clears of
that slice to memclrHasPointers instead of memclrNoHeapPointers.

Updates #17503.

Change-Id: I20750e4bf57f7b8862f3d898bfb32d964b91d07b
Reviewed-on: https://go-review.googlesource.com/31450Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

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

@@ -393,13 +393,11 @@ func memclrrange(n, v1, v2, a *Node) bool {
 		return false
 	}
 
-	// TODO: Use memclrHasPointers if there are pointers.
-
 	// Convert to
 	// if len(a) != 0 {
 	// 	hp = &a[0]
 	// 	hn = len(a)*sizeof(elem(a))
-	// 	memclrNoHeapPointers(hp, hn)
+	// 	memclr{NoHeap,Has}Pointers(hp, hn)
 	// 	i = len(a) - 1
 	// }
 	n.Op = OIF
@@ -425,8 +423,14 @@ func memclrrange(n, v1, v2, a *Node) bool {
 	tmp = conv(tmp, Types[TUINTPTR])
 	n.Nbody.Append(nod(OAS, hn, tmp))
 
+	var fn *Node
+	if haspointers(a.Type.Elem()) {
+		// memclrHasPointers(hp, hn)
+		fn = mkcall("memclrHasPointers", nil, nil, hp, hn)
+	} else {
 		// memclrNoHeapPointers(hp, hn)
-	fn := mkcall("memclrNoHeapPointers", nil, nil, hp, hn)
+		fn = mkcall("memclrNoHeapPointers", nil, nil, hp, hn)
+	}
 
 	n.Nbody.Append(fn)