fix corner case in unused (#1718)

When fixing catch-related issue in #1715, it tries to optimise for duplicate definitions but did not take anonymous functions into account.

Remove such optimisation for now and we can cover this as a more general rule later.

test/compress/drop-unused.js

@@ -931,3 +931,30 @@ issue_1715_3: {
     }
     expect_stdout: "1"
 }
+
+issue_1715_4: {
+    options = {
+        unused: true,
+    }
+    input: {
+        var a = 1;
+        !function a() {
+            a++;
+            try {} catch (a) {
+                var a;
+            }
+        }();
+        console.log(a);
+    }
+    expect: {
+        var a = 1;
+        !function() {
+            a++;
+            try {} catch (a) {
+                var a;
+            }
+        }();
+        console.log(a);
+    }
+    expect_stdout: "1"
+}