fix corner case in reduce_vars

fixes #4218

lib/compress.js

@@ -993,6 +993,7 @@ merge(Compressor.prototype, {
                     return node.value;
                 };
                 d.fixed.assigns = [ node ];
+                if (node.name instanceof AST_SymbolConst && d.redefined()) d.single_use = false;
             } else {
                 d.fixed = false;
             }

test/compress/const.js

@@ -931,6 +931,30 @@ issue_4207: {
     expect_stdout: "0"
 }
 
+issue_4218: {
+    options = {
+        reduce_funcs: true,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        {
+            const a = function() {};
+            var b = 0 * a;
+        }
+        console.log(typeof a, b);
+    }
+    expect: {
+        {
+            const a = function() {};
+            var b = 0 * a;
+        }
+        console.log(typeof a, b);
+    }
+    expect_stdout: true
+}
+
 issue_4210: {
     options = {
         reduce_vars: true,