Unify the handling of comparinsons against null and undefined.

Unify the handling of comparinsons against null and undefined.

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