fix corner case in dead_code & ie8 (#3494)

fixes #3493

lib/compress.js

@@ -888,8 +888,10 @@ merge(Compressor.prototype, {
     });
 
     AST_SymbolRef.DEFMETHOD("is_immutable", function() {
-        var orig = this.definition().orig;
+        var def = this.definition();
-        return orig.length == 1 && orig[0] instanceof AST_SymbolLambda;
+        if (def.orig.length != 1) return false;
+        var sym = def.orig[0];
+        return sym instanceof AST_SymbolLambda && def.scope.name === sym;
     });
 
     function is_lhs_read_only(lhs, compressor) {

test/compress/ie8.js

@@ -1887,3 +1887,75 @@ issue_3486_ie8: {
     }
     expect_stdout: "PASS"
 }
+
+issue_3493: {
+    options = {
+        dead_code: true,
+        ie8: false,
+    }
+    input: {
+        var c = "PASS";
+        (function() {
+            try {
+                (function a() {
+                    throw {};
+                })();
+            } catch (a) {
+                a >>= 0;
+                a && (c = "FAIL");
+            }
+        })();
+        console.log(c);
+    }
+    expect: {
+        var c = "PASS";
+        (function() {
+            try {
+                (function a() {
+                    throw {};
+                })();
+            } catch (a) {
+                a >>= 0;
+                a && (c = "FAIL");
+            }
+        })();
+        console.log(c);
+    }
+    expect_stdout: "PASS"
+}
+
+issue_3493_ie8: {
+    options = {
+        dead_code: true,
+        ie8: true,
+    }
+    input: {
+        var c = "PASS";
+        (function() {
+            try {
+                (function a() {
+                    throw {};
+                })();
+            } catch (a) {
+                a >>= 0;
+                a && (c = "FAIL");
+            }
+        })();
+        console.log(c);
+    }
+    expect: {
+        var c = "PASS";
+        (function() {
+            try {
+                (function a() {
+                    throw {};
+                })();
+            } catch (a) {
+                a >>= 0;
+                a && (c = "FAIL");
+            }
+        })();
+        console.log(c);
+    }
+    expect_stdout: "PASS"
+}