Issue Description:

In this code:

import Foundation
func test() -> Bool {
  let a = RunLoop.current
  let b = RunLoop.main
  return a === b
}

We're smart enough to avoid emitting two references to the metadata for NSRunLoop, but not smart enough (in -Onone builds) to avoid emitting the conversion to an ObjC Class twice, which involves a branch. We should be able to reuse the value from the first thick_to_objc_metatype conversion.