swift234

Add a pass to propagate constant array values to array subscript calls.

This allows us to compile the function:

func valueArray() -> Int{
  var a = [1,2,3]
  var r = a[0] + a[1] + a[2]
  return r
}

Down to just a return of the value 6. And should eventually allow us to remove
the overhead of vararg calls.

rdar://19958821