fix corner case in collapse_vars (#3672)

fixes #3671

lib/compress.js

@@ -1367,7 +1367,7 @@ merge(Compressor.prototype, {
                 if (replace_all) return false;
                 return node instanceof AST_SymbolRef
                     && !node.is_declared(compressor)
-                    && !(parent instanceof AST_Assign && parent.left === node);
+                    && !(parent instanceof AST_Assign && parent.operator == "=" && parent.left === node);
             }
 
             function in_conditional(node, parent) {

test/compress/collapse_vars.js

@@ -7464,3 +7464,30 @@ issue_3651: {
     }
     expect_stdout: "PASS"
 }
+
+issue_3671: {
+    options = {
+        collapse_vars: true,
+    }
+    input: {
+        var a = 0;
+        try {
+            a++;
+            A += 0;
+            a = 1 + a;
+        } catch (e) {
+            console.log(a);
+        }
+    }
+    expect: {
+        var a = 0;
+        try {
+            a++;
+            A += 0;
+            a = 1 + a;
+        } catch (e) {
+            console.log(a);
+        }
+    }
+    expect_stdout: "1"
+}