fix compound assignments

lib/compress.js

@@ -688,7 +688,8 @@ merge(Compressor.prototype, {
                         }
                         // Stop immediately if these node types are encountered
                         var parent = tt.parent();
-                        if (node instanceof AST_Debugger
+                        if (node instanceof AST_Assign && node.operator != "=" && lhs.equivalent_to(node.left)
+                            || node instanceof AST_Debugger
                             || node instanceof AST_IterationStatement && !(node instanceof AST_For)
                             || node instanceof AST_SymbolRef && node.undeclared()
                             || node instanceof AST_Try

test/compress/collapse_vars.js

@@ -2167,3 +2167,22 @@ lvalues_def: {
     }
     expect_stdout: true
 }
+
+compound_assignment: {
+    options = {
+        collapse_vars: true,
+    }
+    input: {
+        var a;
+        a = 1;
+        a += a + 2;
+        console.log(a);
+    }
+    expect: {
+        var a;
+        a = 1;
+        a += a + 2;
+        console.log(a);
+    }
+    expect_stdout: "4"
+}