fix corner case in ie8 (#4251)

fixes #4250

lib/compress.js

@@ -5200,7 +5200,10 @@ merge(Compressor.prototype, {
                 var def = sym.definition();
                 if (!def) return;
                 if (def.id in in_use_ids) return;
-                if (def.scope !== self && self.find_variable(sym) === def) return;
+                if (def.scope !== self) {
+                    var d = self.find_variable(sym);
+                    if ((d && d.redefined() || d) === def) return;
+                }
                 log(sym, "Dropping unused loop variable {name}");
                 if (for_ins[def.id] === node) delete for_ins[def.id];
                 var body = [];

test/compress/ie8.js

@@ -2877,3 +2877,30 @@ issue_4235: {
     }
     expect_stdout: "undefined"
 }
+
+issue_4250: {
+    options = {
+        ie8: true,
+        loops: true,
+        unused: true,
+    }
+    input: {
+        console.log(function f() {
+            (function() {
+                for (f in "f");
+            })();
+            return f;
+            var f;
+        }());
+    }
+    expect: {
+        console.log(function f() {
+            (function() {
+                for (f in "f");
+            })();
+            return f;
+            var f;
+        }());
+    }
+    expect_stdout: "0"
+}