Cleaned up nil comparisons in Hydrogen.

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

Committed: http://code.google.com/p/v8/source/detail?r=16266

[danno, 2013-08-09 13:45:11]

lgtm

[Sven Panne, 2013-08-22 06:44:45]

Committed patchset #2 manually as r16266.