swift234

Add a dead code elimination optimization pass.

In a loop like this:
  var j = 2
  for var i = 0; i < 100; ++i {
    j += 3
  }
it will completely eliminate j.

It does not yet support rewriting conditional branches as unconditional
branches in the cases where only empty blocks are control dependent on
an edge. Once this support is added, it will also completely eliminate
the loop itself.

Swift SVN r18615