swift234

Fix malformed IR after constant fold switch enum.

Eventually it crashes in IRGen addIncomingSILArgumentsToPHINodes.
This is exposed by disabling SimplifyCFG and compiling the following
code from repl.swift. The miscompled method is actually
StringCore.extend.
---
protocol Proto {
  func foo()
}
extension Double : Proto {
  func foo() {
    print("Double: \(self)\n")
  }
}
var pr : Proto = 3.14159
---

Swift SVN r21082