feat(minifier): expand fold leading assignments into the var decl (#26142)

expands leading assignment inlining into variable declarations for
`return`, `switch`, `if`, `throw`, `for`, `for in`, `for of`
```js
// return
var a; return a = b(), c;
var a = b(); return c;
// throw
var a; throw a = b(), c;
var a = b(); throw c;
// switch
var a; switch (a = b(), c) {}
var a = b(); switch (c) {}
// for
var a; for (a = b(), c; ;) d;
var a = b(); for (c; ;) d;
// if
var a; if (a = b(), c) d;
var a = b(); if (c) d;
```

fixes #25949