refactor: tidy up compiler

[Simon-He95]

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 main branch for v2.x (or to a previous version 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:

[Justineo]

In general, I'm not a big fan of small refactorings like this when there are no obvious readability issues with the original code, which would make the history of the real logic changes harder to track.

[Simon-He95]

I think this will make the logical distinction more obvious. If it is not satisfied, it will return directly, and it will reduce the situation where the nesting level of many conditions is deep and the code readability is poor.

[Justineo]

I'm not saying that early return is not a useful pattern, and I use it myself. Rather, for code that has been around for a long time and without obvious readability issues, the benefits may not outweigh the costs. The costs include, but are not limited to, the cost of review by maintainers, the additional complexity of tracking changes to the code.

[haoqunjiang]

Responses to previous efforts:
#5085 (comment)
#12559 (comment)

git blame readability before v.s. after this PR:
https://github.com/vuejs/vue/blame/9dd006b481b4299462e044741bac0861c0b1775c/src/compiler/codegen/index.ts#L513-L545
https://github.com/vuejs/vue/blame/bb38547b3d492b658a93fd8fe39e9e57d5c372f1/src/compiler/codegen/index.ts#L513-L546