Remove select_value SIL instruction

[meg-gupta]

This instruction was only created by SimplifyCFG when it could pattern match tests like :

enum E : Int {
  case E0 = 0
  case E1 = 1
  case E2 = 2
  case E3 = 3
}

func makeE(a: Int) -> E {
  return E.fromRaw(a)!
}

We don't need this anymore since LLVM is able to optimize without it.

[meg-gupta]

@swift-ci apple silicon benchmark

[meg-gupta]

@swift-ci test

[atrick]

Thank you! I'm glad this is finally going away. Instructions like this only create complexity and hide optimizer deficiencies