cmd/compile: remove unnecessary write barriers for APPEND

Updates #17330.

Change-Id: I83fe80139a2213f3169db884b84a4c3bd15b58b6
Reviewed-on: https://go-review.googlesource.com/30140
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -667,7 +667,7 @@ func (s *state) stmt(n *Node) {
 				r = s.expr(rhs)
 			}
 		}
-		if rhs != nil && rhs.Op == OAPPEND {
+		if rhs != nil && rhs.Op == OAPPEND && needwritebarrier(n.Left, rhs) {
 			// The frontend gets rid of the write barrier to enable the special OAPPEND
 			// handling above, but since this is not a special case, we need it.
 			// TODO: just add a ptr graying to the end of growslice?