fix return from recursive IIFE (#1570)

`side-effects` did not account for IIFEs being able to reference itself thus making its return value potentially significant

test/compress/issue-1569.js

@@ -0,0 +1,19 @@
+inner_reference: {
+    options = {
+        side_effects: true,
+    }
+    input: {
+        !function f(a) {
+            return a && f(a - 1) + a;
+        }(42);
+        !function g(a) {
+            return a;
+        }(42);
+    }
+    expect: {
+        !function f(a) {
+            return a && f(a - 1) + a;
+        }(42);
+        !void 0;
+    }
+}