cmd/compile: use t.Key() instead of t.Down

This was the only unconverted instance.

Change-Id: Ic0ba75824614fcd1e055316e62e26acd06801dd1
Reviewed-on: https://go-review.googlesource.com/21247
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -859,7 +859,7 @@ func maplit(ctxt int, n *Node, var_ *Node, init *Nodes) {
 	if b != 0 {
 		// build type [count]struct { a Tindex, b Tvalue }
 		t := n.Type
-		tk := t.Down
+		tk := t.Key()
 		tv := t.Type
 
 		syma := Lookup("a")