Fix regression around type skolems and if exprs

[gnovark]

Not sure what the actual cause is, but it seems to be triggered by containsForeignJumps spuriously returning true due to not checking to see if the symbol is actually a label.

[retronym]

This fix looks to be correct, as it we are currently creating needless intermediate states for if/else without an await.

But it just masks the an underlying problem. The type error with skolems resurfaces with:

  def run(k: K) = async {
    val res = await(result(k.f))
    if(true) {
      println(res)
      await(result(k.f))
    }
    res
  }

[retronym]

I'm looking into the underlying problem.

[retronym]

A smaller test case:

import scala.async.internal.AsyncId._
class F[A]
class S[A]
def result[A](f: F[A]): S[A with String] = ???
val f: F[_] = ???
val s /* F[$1 with String] */ = result(f)
async {
  val res = await(s)
  if(true) {
    await(0)
  }
  res
}

 found   : S#10272[_$1#10308 with String#137] where type _$1#10308
 required: S#10272[_$1#10311 with String#137] forSome { type _$1#10311 }

I can probably brute force this by generating casts if the types of a synthetic Assign doesn't line up.