Merge the code of the event modifier

Created by: VICTORYGS

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information: Test Demo

       new Vue({
                el: '.aa',
                template: `
          <div>
            <button @click.ctrl.stop>click.ctrl.stop</button>
            <button @click.alt.shift.stop="foo">click.alt.shift.stop="foo"</button>
            <p>
                keyup.shift.stop.exact.k="foo"
                <input @keyup.shift.stop.exact.k="foo"/>
            </p>
          </div>
        `,
                methods: {
                    foo(e) {
                        console.log(e);
                    }
                }
            })

The guard codes corresponding to all modifiers no longer need to be executed(if(...)return) separately.