Loads the constructor of a JS class (native or not).
cls must represent a non-trait JS class (native or not).
This is used typically to instantiate a JS class, and most importantly
if it is a Scala.js-defined JS class. Given the class
class Foo(x: Int) extends js.Object
The instantiation new Foo(1) would be represented as
This node is also useful to encode o.isInstanceOf[Foo]:
JSBinaryOp(instanceof, o, LoadJSConstructor(ClassType("Foo")))
If Foo is Scala.js-defined, the presence of this node makes it
instantiable, and therefore reachable.