feat(es/minifier): Reuse variable names

[kdy1]

Description:

• Node APIs (from google closure compiler)

• Control flow graph

• DataFlow analyzer

This has some TODOs left

• Liveness analyzer

• Coalesce variable names pass

Related issue (if exists):

[kdy1]

@Austaras Can you describe more about reusing variable names in #2934 (comment) ?
I tried implementing it but seems like the size difference is too small.

One thing I found from diffing is reuse of variables in non-first statements, like

// lots of code

var a = b + b;

for (var d = init;) {
    use(d)
}

became

// lots of code

var a = b + b;

for (b = init;) {
    use(b)
}

but, I'm curious if there was more renaming. (I didn't analyze fully yet)

[Austaras]

Seems like it's not really about initializer, but about reference

function hello(name) {
  console.log('Hello, ' + name);
  var b = window.a
  console.log(b === b)
}

be turned into

function hello(a) {
  console.log("Hello, " + a);
  a = window.a;
  console.log(a === a);
}

so a is not referred after var b declaration, so it can be reused

[kdy1]

Thank you!

[kdy1]

Closing in favor of #5781