Fix an issue with new update-expression tests.

packages/babel-plugin-transform-modules-commonjs/test/fixtures/update-expression/negative-suffix/input.js

@@ -0,0 +1,7 @@
+export let diffLevel = 0;
+
+export function diff() {
+  if (!--diffLevel) {
+    console.log("hey");
+  }
+}

packages/babel-plugin-transform-modules-commonjs/test/fixtures/update-expression/negative-suffix/output.js

@@ -0,0 +1,15 @@
+"use strict";
+
+Object.defineProperty(exports, "__esModule", {
+  value: true
+});
+exports.diff = diff;
+exports.diffLevel = void 0;
+let diffLevel = 0;
+exports.diffLevel = diffLevel;
+
+function diff() {
+  if (!(exports.diffLevel = diffLevel = diffLevel - 1)) {
+    console.log("hey");
+  }
+}