Introduce macro assembler support for pushing heap-object handles and
for comparing a register to a heap-object handle.

Cleanup: Remove some code that was needlessly commented out.  Added
TODO's for code that was commented out (where we do not hit an
UNIMPLEMENTED). 
Review URL: http://codereview.chromium.org/140060

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2241 ce2b1a6d-e550-0410-aec6-3dcde31c8c00