Clarify / fix nested generators

[NTillmann]

Currently, the front end may produces generator trees that are actually DAGs --- the same generator may appear multiple times.

The back end was designed assuming that generators form a tree, not a DAG. Note this currently disabled invariant:
https://github.com/facebook/prepack/blob/c3a1fd3c46a5181dc1288f121b576cc4001d002f/src/serializer/ResidualHeapVisitor.js#L1055

With the invariant disabled, the visitor and serializer will simply visit each multi-referenced generator multiple times. I think this might just work fine, as long as such a generator doesn't declare derived abstract values - if it does, I suspect that it will confuse the waiting logic in the emitter.

The way things are right now, the generated code is suboptimal, and looking at it, it's not clear to me if the front-end produces this by design or by error.

Consider the following program, a variation of test/serializer/abstract/Throw6.js:

(function() {
    let x = global.__abstract ? __abstract("boolean", "true") : true;
    function foo(b) {
        if (b) throw new Error("" + Date.now());
        return "is false";
    }
    let z = foo(!x);
    inspect = function() { return z; }
})();

It currently "works", prepacking to the following code:

(function () {
  var $$0 = { enumerable: false, configurable: true, writable: true };
  var _$1 = this;
  var _$2 = _$1.Error;
  var _$3 = _$2.prototype;
  var _$4 = _$1.Object;
  var _$5 = _$4.defineProperty;
  var _8 = function () { return "is false"; };
  var __constructor = function () {};
  var _1 = true;
  var _0 = !_1;
  if (_0) {
    var _$0 = _$1.Date.now();
  }
  if (_0) {
    var _$0 = _$1.Date.now();
  } else {
    inspect = _8;
  }
  var _7 = _$3;
  var _2 = (__constructor.prototype = _7, new __constructor());
  $$0.value = "Error\n    at foo (unknown)\n    at /tmp/test.js:7:13\n    at /tmp/test.js:1:1", _$5(_2, "stack", $$0);
  var _4 = "" + _$0;
  $$0.value = _4, _$5(_2, "message", $$0);
  if (_0) throw _2;else ;
}).call(this);

Note the redundant if (_0) checks and the redundant Date.now() calls. Is this really what the front-end wanted to generate?

[hermanventer]

Since there is no catch or finally handler in the picture, no generator should appear more than once, so this is clearly not what we want.