Change code pointer in function objects to a pointer to the first

Change code pointer in function objects to a pointer to the first
instruction.

By changing the pointer to the code object to a pointer to the first
instruction we can call directly this instruction directly instead of
looking up the address through the code object.

Review URL: http://codereview.chromium.org/3156028

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5309 ce2b1a6d-e550-0410-aec6-3dcde31c8c00