swift234

During DCE, eliminate branches to dead regions of code.

Enhances DCE to make unreachable those regions of code that have no
effect.

This allows loops like:
  for i in 0..n {
    // do nothing
  }
to be eliminated by first running DCE to make the loop unreachable, and
the CFG simplification to actually delete the blocks that make up the
loop (assuming we're talking -Ofast and cond_fails have been removed).

What's especially nice is that this can make unreachable several levels
of dead code, including deleting the code that produces the values used
to conditionally branch to other dead code, all in a single pass rather
than needing to iterate between DCE and CFG simplification to achieve
the same effect. For example, this:

  func f(b: Bool, c: Bool, d: Bool) {
    if (b && c) {
      // nothing useful here
      if (c && d) {
        // nothing useful here
        if (b && d) {
          // nothing useful here
        }
      }
    }
  }

is effectively reduced to:
  func f(b: Bool, c: Bool, d: Bool) {
    goto end      // pretend for a second we have goto
    if (b && c) {
      // nothing useful here
      if (c && d) {
        // nothing useful here
        if (b && d) {
          // nothing useful here
        }
      }
    }
    end:
  }
after a single pass, after which unreachable code elimination reduces
this to:
  func f(b: Bool, c: Bool, d: Bool) {
  }

Swift SVN r18664