mapAppend = right.(*ir.CallExpr)
		if !ir.SameSafeExpr(left, mapAppend.Args[0]) {
			base.Fatalf("not same expressions: %v != %v", left, mapAppend.Args[0])
		}
	}

	left = walkExpr(left, init)
	left = safeExpr(left, init)
	if mapAppend != nil {
		mapAppend.Args[0] = left
	}

	if n.Op() == ir.OASOP {
		// Rewrite x op= y into x = x op y.