fix corner case in reduce_vars (#5435)

fixes #5434

lib/compress.js

@@ -649,7 +649,7 @@ Compressor.prototype.compress = function(node) {
             }
             if (!HOP(tw.safe_ids, def.id)) {
                 if (!safe) return false;
-                if (safe.read) {
+                if (safe.read || tw.in_loop) {
                     var scope = tw.find_parent(AST_BlockScope);
                     if (scope instanceof AST_Class) return false;
                     if (def.scope.resolve() !== scope.resolve()) return false;

test/compress/reduce_vars.js

@@ -7861,3 +7861,38 @@ issue_5324: {
     }
     expect_stdout: "NaN"
 }
+
+issue_5434: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        console.log(function(a) {
+            for (var i = 0; i < 2; i++) {
+                var b = "FAIL";
+                f && f();
+                a = b;
+                var f = function() {
+                    b = "PASS";
+                };
+            }
+            return a;
+        }());
+    }
+    expect: {
+        console.log(function(a) {
+            for (var i = 0; i < 2; i++) {
+                var b = "FAIL";
+                f && f();
+                a = b;
+                var f = function() {
+                    b = "PASS";
+                };
+            }
+            return a;
+        }());
+    }
+    expect_stdout: "PASS"
+}