if/switch expressions aren't allowed with an optional chain assignment

[hamishknight]

The following gives an error:

struct S {
  var x: Int
}
func foo(_ s: S?) {
  var s = s
  s?.x = if .random() { 0 } else { 1 }
  // error: 'if' may only be used as expression in return, throw, or as the source of an assignment 
}

But we ought to allow it.

rdar://109305454