Issue Description:

The following code snippet crashes the compiler with "Illegal instruction: 4" in Swift 3 mode and Swift 4 mode:

let a = (0...1).flatMap { _ in return nil }.max() ?? 0

Also adding more type information doesn't fix the crash:

let a = (0...1).flatMap { i in
  if i % 2 == 0 {
    return nil
  } else {
    return i
  }
}.max() ?? 0

Both successfully compile in Swift 3.1

As a workaround, adding type hints fixes the problem:

let a = (0...1).flatMap { i -> Int? in
  if i % 2 == 0 {
    return nil
  } else {
    return i
  }
}.max() ?? 0

Additional info: This bug was introduced in Xcode beta 5. In beta 4 the compiler didn't crash.