Improve dominator computation to avoid worst-case quadratic time.

Improve dominator computation to avoid worst-case quadratic time.

In case of a degenerated CFG like in the example below processing
predecessors in the wrong order yields n^2 runtime.

  do {
    if (x) break;
    if (x) break;
    if (x) break;
    if (x) break;
    if (x) break;
    if (x) break;
    if (x) break;
    if (x) break;
    if (x) break;
    if (x) break;
    if (x) break;
    // etc.
  } while (false);

Reversing iteration order avoids this.
Review URL: http://codereview.chromium.org/8502012

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9905 ce2b1a6d-e550-0410-aec6-3dcde31c8c00