cmd/compile: len(n.List.Slice()) -> n.List.Len()

Minor cleanup.

This is the only such instance in the compiler.

Change-Id: I4e8ecde57d71867c7e1ac4d17e2154a91dd262b0
Reviewed-on: https://go-review.googlesource.com/39209
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -927,7 +927,7 @@ func maplit(n *Node, m *Node, init *Nodes) {
 	nerr := nerrors
 
 	a := nod(OMAKE, nil, nil)
-	a.List.Set2(typenod(n.Type), nodintconst(int64(len(n.List.Slice()))))
+	a.List.Set2(typenod(n.Type), nodintconst(int64(n.List.Len())))
 	litas(m, a, init)
 
 	// count the initializers