Simplify after DCE (Multiple Passes)

[boopathi]

Related #61 , #317

Right now DCE is run in a single pass at the Program.exit. But this blocks other optimisations that could be performed after DCE - such as simplify.

an example - from #420 ... if_return optimisation (simplify) after DCE is not happening here because DCE is at the end, and requires a second pass.

Also, if the function is an arrow function, after DCE, an arrow function with a single return statement could be further simplified to remove the BlockStatement, which is also not happening now because of the same reasons.

// input
function foo(...args) {
  if (!i) return;
  else {
    const x = {args};
    while(false);
    return x;
  }
};

// out
function foo(...a) {
  if (!i) return;
  return { args: a };
}

// could reduce to
function foo(...a) {
  if (i) return {args: a};
}