fix corner case in collapse_vars (#3701)

fixes #3700

lib/compress.js

@@ -1395,7 +1395,7 @@ merge(Compressor.prototype, {
                     var rhs_fn = scan_rhs;
                     for (var i = 0; !abort && i < fn.body.length; i++) {
                         var stat = fn.body[i];
-                        if (stat instanceof AST_Exit) {
+                        if (stat instanceof AST_Return) {
                             if (stat.value) stat.value.transform(scanner);
                             break;
                         }

test/compress/collapse_vars.js

@@ -7734,3 +7734,34 @@ issue_3698_3: {
     }
     expect_stdout: "2"
 }
+
+issue_3700: {
+    options = {
+        collapse_vars: true,
+    }
+    input: {
+        var a = "FAIL";
+        try {
+            a = "PASS";
+            (function() {
+                throw 0;
+            })();
+            a = 1 + a;
+        } catch (e) {
+        }
+        console.log(a);
+    }
+    expect: {
+        var a = "FAIL";
+        try {
+            a = "PASS";
+            (function() {
+                throw 0;
+            })();
+            a = 1 + a;
+        } catch (e) {
+        }
+        console.log(a);
+    }
+    expect_stdout: "PASS"
+}