Cleaned up nil comparisons in Hydrogen.

This consists basically of 2 parts:

   * Restructured BuildCompareNil to make the logic behind it clearer.
     Note that it is intentionally written in a quite stylized way.

   * Replaced a usesless IfBuilder by a plain HInstruction, removing
     some empty blocks.

R=danno@chromium.org

Review URL: https://codereview.chromium.org/22305004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16266 ce2b1a6d-e550-0410-aec6-3dcde31c8c00