Fix TypeEncoding losing side-effects when wrapping Unit-typed expression

[gsps]

TypeEncoding may drop Unit-typed expressions when wrapping them in an adaptation to Object:

object TypeEncodingEatsTrees {
  def foo(b: Boolean): Unit = {
    if (b) {
      true
    } else {
      assert(false)
      ()
    }

    ()
  }
}

The problematic transformation step:

-------------Functions-------------
def foo(b: Boolean): Unit = {
  val t: Any = if (b) {
    true
  } else {
    assert(false)
    ()
  }
  val res: Any = t
  val tmp: Any = res
  ()
}


Symbols after TypeEncoding

-------------Functions-------------
def foo(b: Boolean): Unit = {
  val t: Object = if (b) {
    Boolean(true)
  } else {
    Unit()
  }
  val res: Object = t
  val tmp: Object = res
  ()
}

What's needed is a fix for wrap that's analogous to the changes to unwrap in #818. Could've noticed that right away, my bad.