Rule proposal: `no-useless-assign`

Description

Got the idea from https://lgtm.com/projects/g/prettier/prettier/snapshot/dae9b0317dae184854caf66c44eec17a95200e98/files/src/language-js/parse/postprocess/typescript.js?sort=name&dir=ASC&mode=heatmap#L66

I don't think no-unused-vars rule should check this case, because the variable is actually "used" in some case, only should not assign when it's a last use.

Fail

function foo(bar) {
  bar = doSomething();
}

function foo(bar) {
  bar = doSomething(bar);
}

function foo(bar) {
  bar = doSomething(bar);

  // ... do something else, but not using bar
}

Pass

function foo(bar) {
  bar = doSomething(bar);

  return bar;
}

function foo(bar) {
  bar = doSomething(bar);

  use(bar);
}

function foo(bar) {
  function aInnerFunctionUsesBar() {
   return bar;
  }

  bar = doSomething(bar);

  // bar is not used but calling another function uses bar
  aInnerFunctionUsesBar()
}