feat(es/minifier): Optimize switch with side effect and termination tests

CPunisher commented 4 weeks ago

Description:

This is learnt from https://github.com/terser/terser/pull/1044 Key point: all the cases (statements) after the last side-effect and non-terminated one are useless.

There are also some points on whether a statement is side-effect free:

Var declaration: I think var declaration always contains side effect.
Function declaration: In non-strict mode, function declaration is same as var declaration. In strict mode, function is declared in enclosing block scope, so it's side-effect free. But there is a bad case when we call the function before it is declared in the switch case:
```
"use strict";
const k = (() => {
let x = 1;
switch (x) {
    case y():
    case x: {
        async function y() {
            console.log(123);
        }
    }
}
return x;
})();
```

This is an existing bug. I'm not sure whether we should fix it since it is a very bad code and I also find some similar bad cases with terser.

~I'm also not sure it's appropriate to introduce context variable in_strict for StmtExt:: may_have_side_effects, because in_strict could change from false to true. But this is a conservative mistake and does not break the program. Maybe we should use visitor for context-aware side effect checker in the future.~

Related issue: