Rather than passing in a pair of unequal-length lists, pass the default
subgraph separately.  Construct the typecase from the top down rather than
the bottom up, so it doesn't need an intermediate zone list.

Also, change a basic block's 'last' instruction field to really be its last
instruction by correctly updating it when inserting and removing
instructions.

Review URL: http://codereview.chromium.org/6516016

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6783 ce2b1a6d-e550-0410-aec6-3dcde31c8c00