swift234

Add a small peephole for removing alloc_existential_box that are initialized but never used for anything.

After inlining of functions that throw, in some cases, we can get rid of the exception object. With this change we
can remove all error handling overhead from the code below:

func foo() throws {
  do {
    try callee()
  } catch {
  }
}

func callee() throws {
  throw VendingMachineError.OutOfStock
}

Swift SVN r30018