cmd/compile: temporarily disable inplace append special case

Fixes #15246
Re-opens #14969

Change-Id: Ic0b41c5aa42bbb229a0d62b7f3e5888c6b29293d
Reviewed-on: https://go-review.googlesource.com/21891
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -699,7 +699,8 @@ func (s *state) stmt(n *Node) {
 				// If the slice can be SSA'd, it'll be on the stack,
 				// so there will be no write barriers,
 				// so there's no need to attempt to prevent them.
-				if samesafeexpr(n.Left, rhs.List.First()) && !s.canSSA(n.Left) {
+				const doInPlaceAppend = false // issue 15246
+				if doInPlaceAppend && samesafeexpr(n.Left, rhs.List.First()) && !s.canSSA(n.Left) {
 					s.append(rhs, true)
 					return
 				}