fix reduce_vars on single AST_Defun reference across loop (#2593)

lib/compress.js

@@ -334,6 +334,11 @@ merge(Compressor.prototype, {
                 if (node instanceof AST_SymbolRef) {
                     var d = node.definition();
                     d.references.push(node);
+                    if (d.references.length == 1
+                        && !d.fixed
+                        && d.orig[0] instanceof AST_SymbolDefun) {
+                        loop_ids[d.id] = in_loop;
+                    }
                     var value;
                     if (d.fixed === undefined || !safe_to_read(d) || d.single_use == "m") {
                         d.fixed = false;
@@ -402,9 +407,9 @@ merge(Compressor.prototype, {
                         d.fixed = false;
                     } else {
                         d.fixed = node;
+                        d.single_use = ref_once(d);
                         loop_ids[d.id] = in_loop;
                         mark(d, true);
-                        d.single_use = ref_once(d);
                     }
                     var save_ids = safe_ids;
                     safe_ids = Object.create(null);

test/compress/reduce_vars.js

@@ -4838,3 +4838,31 @@ inverted_var: {
     }
     expect_stdout: true
 }
+
+defun_single_use_loop: {
+    options = {
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        for (var x, i = 2; --i >= 0; ) {
+            var y = x;
+            x = f;
+            console.log(x === y);
+        }
+        function f() {};
+    }
+    expect: {
+        for (var x, i = 2; --i >= 0; ) {
+            var y = x;
+            x = f;
+            console.log(x === y);
+        }
+        function f() {};
+    }
+    expect_stdout: [
+        "false",
+        "true",
+    ]
+}