fix corner case in reduce_vars (#5047)

fixes #5046

lib/compress.js

@@ -1000,11 +1000,7 @@ merge(Compressor.prototype, {
                     arg.walk(tw);
                     if (arg instanceof AST_Spread) iife = false;
                 });
-                if (iife) {
-                    exp.reduce_vars = reduce_iife;
-                } else {
-                    exp.safe_ids = tw.safe_ids;
-                }
+                if (iife) exp.reduce_vars = reduce_iife;
                 exp.walk(tw);
                 if (iife) delete exp.reduce_vars;
                 return true;
@@ -1299,9 +1295,7 @@ merge(Compressor.prototype, {
                 node.expressions.forEach(function(exp) {
                     exp.walk(tw);
                 });
-                tag.safe_ids = tw.safe_ids;
                 tag.walk(tw);
-                delete tag.reduce_vars;
                 return true;
             }
             tag.walk(tw);

test/compress/functions.js

@@ -6285,3 +6285,34 @@ issue_5036: {
     }
     expect_stdout: "PASS"
 }
+
+issue_5046: {
+    options = {
+        conditionals: true,
+        evaluate: true,
+        keep_fnames: true,
+        passes: 2,
+        reduce_vars: true,
+        side_effects: true,
+        toplevel: true,
+    }
+    input: {
+        var a = 0;
+        if (a)
+            0();
+        else
+            (function f() {
+                f;
+                return a = "PASS";
+            })();
+        console.log(a);
+    }
+    expect: {
+        var a = 0;
+        (a ? 0 : function f() {
+            return a = "PASS";
+        })();
+        console.log(a);
+    }
+    expect_stdout: "PASS"
+}