Add a premonomorphic state to the call target cache.

Add a premonomorphic state to the call target cache.

This allows access to lazily initialized constructors and functions
fetched through the context chain to remain monomorphic.

  (function() {
    var fn = function() {
      fn = function() { return 42; };
      return fn();
    }
    for (var i = 0; i < 100; i++) {
      fn();
    }
  })();