drop unused assignemnts

[Constellation]

Drop unused assignments

function test() {
  var i;
  i = 20;
}

to

function test() {
  var i;
  20;
}

and then

function test() {
  var i;
}

and finally

function test() { }

[Constellation]

My model is following.

• construct CFG
• assign variable slots to basic block
• numbering assigned variable (SSA-like)
• track numbered definition and when use-list is empty, we can remove this definition

[Constellation]

We can start it with simple case.
If complicated pattern is found (such as try-catch-finally), we can give up constructing CFG on this function, like optimizing compiler does.