cmd/compile: fix OADDSTR buffer size calculation

The size calculation has been wrong since this code was first committed
in https://golang.org/cl/3120.  The effect was that the compiler always
allocated a temporary buffer on the stack for a non-escaping string
concatenation.  This turns out to make no practical difference, as the
compiler always allocates a buffer of the same size (32 bytes) and the
runtime only uses the temporary buffer if the concatenated strings
fit (check is in rawstringtmp in runtime/string.go).

The effect of this change is to avoid generating a temporary buffer on
the stack that will not be used.

Change-Id: Id632bfe3d6c113c9934c018a2dd4bcbf1784a63d
Reviewed-on: https://go-review.googlesource.com/20112
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -2718,8 +2718,8 @@ func addstr(n *Node, init **NodeList) *Node {
 	if n.Esc == EscNone {
 		sz := int64(0)
 		for l := n.List; l != nil; l = l.Next {
-			if n.Op == OLITERAL {
-				sz += int64(len(n.Val().U.(string)))
+			if l.N.Op == OLITERAL {
+				sz += int64(len(l.N.Val().U.(string)))
 			}
 		}