Although this patch is not small, most parts of it are rather mechanical:

 * First of all, the concept of a 'nil-like' value is introduced, which can be
   null or undefined. They are treated symmetrically regarding comparisons, so
   it makes sense to handle them in a uniform manner. It is a mystery why
   JavaScript defines two of those beasts, when even *one* is a design wart...

 * Extended and renamed a few things which now handle undefined in addition to
   null.

 * Made the parts of the full code generator and the hydrogen generation which
   deal with comparisons a bit more similar regarding their handling of special
   cases.

 * Refactored the syntactical detection of special cases for comparisons,
   hopefully making them a bit more readable and less copy-n-paste-oriented.
   Things like this should really be a one-liner in any sane programming
   language... :-P

 * Cut down the length of the argument lists of a few functions to something
   less insane, making them more easily understandable locally. This involves
   minor code duplication, but this was a good tradeoff and can be remedied
   later if necessary.

 * Replaced some boolean arguments with more readable enums.

 * Fixed a TODO: Values which are definitely a Smi or unboxed can never be equal
   to null or undefined.
Review URL: http://codereview.chromium.org/7918012

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9323 ce2b1a6d-e550-0410-aec6-3dcde31c8c00