cmd/compile: minor cleanup in mapinit

Change-Id: I7d58d200f7e8b2c0a6e35371da0dafd9b44e9057
Reviewed-on: https://go-review.googlesource.com/26757
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -974,13 +974,14 @@ func maplit(ctxt int, n *Node, m *Node, init *Nodes) {
 			val = temp(m.Type.Val())
 		}
 
-		setlineno(r.Left)
-		a = Nod(OAS, key, r.Left)
+		setlineno(index)
+		a = Nod(OAS, key, index)
 		a = typecheck(a, Etop)
 		a = walkstmt(a)
 		init.Append(a)
-		setlineno(r.Right)
-		a = Nod(OAS, val, r.Right)
+
+		setlineno(value)
+		a = Nod(OAS, val, value)
 		a = typecheck(a, Etop)
 		a = walkstmt(a)
 		init.Append(a)