This change adds a new IR instruction for polymorphic loads. It performs
map compares and loads in one IR instruction instead of splitting each
load into a graph of map-compares and field loads.
 
The advantage is a smaller IR and less basic blocks, plus it allows to
do GVN on polymorphic loads.
Review URL: http://codereview.chromium.org/6708085

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7336 ce2b1a6d-e550-0410-aec6-3dcde31c8c00