fix corner case in merge_vars (#5771)

fixes #5770

test/compress/merge_vars.js

@@ -3889,3 +3889,56 @@ issue_5714: {
         "42",
     ]
 }
+
+issue_5770_1: {
+    options = {
+        dead_code: true,
+        loops: true,
+        merge_vars: true,
+        toplevel: true,
+    }
+    input: {
+        L: do {
+            if (console)
+                for (var a = "FAIL 1"; a; a--)
+                    continue L;
+            var b = "FAIL 2";
+        } while (console.log(b || "PASS"));
+    }
+    expect: {
+        L: do {
+            if (console) {
+                var a = "FAIL 1";
+                if (a)
+                    continue L;
+            }
+            var b = "FAIL 2";
+        } while (console.log(b || "PASS"));
+    }
+    expect_stdout: "PASS"
+}
+
+issue_5770_2: {
+    options = {
+        conditionals: true,
+        dead_code: true,
+        if_return: true,
+        loops: true,
+        merge_vars: true,
+        toplevel: true,
+    }
+    input: {
+        L: do {
+            for (var a = "FAIL 1"; a; a--)
+                continue L;
+            var b = "FAIL 2";
+        } while (console.log(b || "PASS"));
+    }
+    expect: {
+        L: do {
+            var a = "FAIL 1";
+            var b;
+        } while (a || (b = "FAIL 2"), console.log(b || "PASS"));
+    }
+    expect_stdout: "PASS"
+}

test/reduce.js

@@ -363,9 +363,17 @@ module.exports = function reduce_test(testcase, minify_options, reduce_options)
                 node.condition,
                 node.body,
                 node.alternative,
-            ][ (node.start._permute * steps | 0) % 3 ];
+                node,
+            ][ (node.start._permute * steps | 0) % 4 ];
             node.start._permute += step;
-            if (expr) {
+            if (expr === node) {
+                if (node.alternative) {
+                    expr = node.clone();
+                    expr.alternative = null;
+                    CHANGED = true;
+                    return expr;
+                }
+            } else if (expr) {
                 // replace if statement with its condition, then block or else block
                 CHANGED = true;
                 return to_statement(expr);