cmd/compile: cgen_append can handle complex targets

Post-liveness fix, the slices on both sides can now be
indirects of & variables.  The cgen code handles those
cases just fine.

Fixes #15988

Change-Id: I378ad1d5121587e6107a9879c167291a70bbb9e4
Reviewed-on: https://go-review.googlesource.com/23863
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -2855,11 +2855,6 @@ func cgen_append(n, res *Node) {
 		Dump("cgen_append-n", n)
 		Dump("cgen_append-res", res)
 	}
-	if res.Op != ONAME && !samesafeexpr(res, n.List.First()) {
-		Dump("cgen_append-n", n)
-		Dump("cgen_append-res", res)
-		Fatalf("append not lowered")
-	}
 	for _, n1 := range n.List.Slice() {
 		if n1.Ullman >= UINF {
 			Fatalf("append with function call arguments")

test/fixedbugs/issue15988.go

+// compile
+
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f(p, q []int) {
+	p = append(q, 5)
+	sink = &p
+}
+
+var sink *[]int