improve collapse_vars on side-effect-free replacements (#2583)

test/compress/collapse_vars.js

@@ -3805,3 +3805,51 @@ may_throw: {
         }
     }
 }
+
+side_effect_free_replacement: {
+    options = {
+        collapse_vars: true,
+        inline: true,
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        var b;
+        (function(a) {
+            x(a);
+        })(b);
+    }
+    expect: {
+        var b;
+        x(b);
+    }
+}
+
+recursive_function_replacement: {
+    rename = true
+    options = {
+        collapse_vars: true,
+        inline: true,
+        passes: 2,
+        reduce_funcs: true,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    mangle = {}
+    input: {
+        function f(a) {
+            return x(g(a));
+        }
+        function g(a) {
+            return y(f(a));
+        }
+        console.log(f(c));
+    }
+    expect: {
+        function f(n) {
+            return x(y(f(n)));
+        }
+        console.log(f(c));
+    }
+}