fix corner case in ie8 & reduce_vars (#4029)

fixes #4028

lib/compress.js

@@ -600,6 +600,7 @@ merge(Compressor.prototype, {
                         return node.right;
                     };
                 } else {
+                    if (d.single_use) d.single_use = false;
                     sym.fixed = d.fixed = function() {
                         return make_node(AST_Binary, node, {
                             operator: node.operator.slice(0, -1),
@@ -894,9 +895,9 @@ merge(Compressor.prototype, {
             d.assignments++;
             var fixed = d.fixed;
             if (safe_to_read(tw, d) && safe_to_assign(tw, d)) {
-                if (d.single_use) d.single_use = false;
                 push_ref(d, exp);
                 mark(tw, d);
+                if (d.single_use) d.single_use = false;
                 d.fixed = function() {
                     return make_node(AST_Binary, node, {
                         operator: node.operator.slice(0, -1),

test/compress/ie8.js

@@ -2661,3 +2661,33 @@ issue_4019: {
     }
     expect_stdout: "0"
 }
+
+issue_4028: {
+    options = {
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    mangle = {
+        ie8: true,
+    }
+    input: {
+        function a() {
+            try {
+                A;
+            } catch (e) {}
+        }
+        var b = a += a;
+        console.log(typeof b);
+    }
+    expect: {
+        function a() {
+            try {
+                A;
+            } catch (a) {}
+        }
+        var b = a += a;
+        console.log(typeof b);
+    }
+    expect_stdout: "string"
+}